diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 039a05f..f0ac704 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -3508,6 +3508,11 @@
 in the path name. This is useful to implement suggestion services
 finding a file by partial name.
 
+The integer-valued request parameter `parent` changes the response to return a
+list of the files which are different in this commit compared to the given
+parent commit. This is useful for supporting review of merge commits.  The value
+is the 1-based index of the parent's position in the commit object.
+
 .Request
 ----
   GET /changes/myProject~master~I8473b95934b5732ac55d26311a706c9c2bde9940/revisions/674ac754f91e64a0efb8087e59a176484bd534d1/files/?reviewed HTTP/1.0
@@ -3753,6 +3758,11 @@
 The `base` parameter can be specified to control the base patch set from which the diff should
 be generated.
 
+The integer-valued request parameter `parent` can be specified to control the
+parent commit number against which the diff should be generated.  This is useful
+for supporting review of merge commits.  The value is the 1-based index of the
+parent's position in the commit object.
+
 [[weblinks-only]]
 If the `weblinks-only` parameter is specified, only the diff web links are returned.
 
@@ -4297,6 +4307,9 @@
 The side on which the comment was added. +
 Allowed values are `REVISION` and `PARENT`. +
 If not set, the default is `REVISION`.
+|`parent`      |optional|
+The 1-based parent number. Used only for merge commits when `side == PARENT`.
+When not set the comment is for the auto-merge tree.
 |`line`        |optional|
 The number of the line for which the comment was done. +
 If range is set, this equals the end line of the range. +
diff --git a/gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/AbstractDaemonTest.java b/gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/AbstractDaemonTest.java
index 1b325ec..a157040 100644
--- a/gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/AbstractDaemonTest.java
+++ b/gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/AbstractDaemonTest.java
@@ -19,9 +19,13 @@
 import static com.google.gerrit.extensions.api.changes.SubmittedTogetherOption.NON_VISIBLE_CHANGES;
 import static com.google.gerrit.server.group.SystemGroupBackend.REGISTERED_USERS;
 
+import static org.eclipse.jgit.lib.Constants.HEAD;
+
 import com.google.common.base.Function;
 import com.google.common.base.Optional;
 import com.google.common.base.Strings;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Sets;
 import com.google.common.primitives.Chars;
@@ -490,6 +494,29 @@
     return result;
   }
 
+  protected PushOneCommit.Result createMergeCommitChange(String ref)
+      throws Exception {
+    ObjectId initial = repo().exactRef(HEAD).getLeaf().getObjectId();
+
+    PushOneCommit.Result p1 = pushFactory.create(db, admin.getIdent(),
+        testRepo, "parent 1", ImmutableMap.of("foo", "foo-1", "bar", "bar-1"))
+        .to(ref);
+
+    // reset HEAD in order to create a sibling of the first change
+    testRepo.reset(initial);
+
+    PushOneCommit.Result p2 = pushFactory.create(db, admin.getIdent(),
+        testRepo, "parent 2", ImmutableMap.of("foo", "foo-2", "bar", "bar-2"))
+        .to(ref);
+
+    PushOneCommit m = pushFactory.create(db, admin.getIdent(), testRepo, "merge",
+        ImmutableMap.of("foo", "foo-1", "bar", "bar-2"));
+    m.setParents(ImmutableList.of(p1.getCommit(), p2.getCommit()));
+    PushOneCommit.Result result = m.to(ref);
+    result.assertOkStatus();
+    return result;
+  }
+
   protected PushOneCommit.Result createDraftChange() throws Exception {
     return pushTo("refs/drafts/master");
   }
diff --git a/gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/PushOneCommit.java b/gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/PushOneCommit.java
index 7179e80..4589fe6 100644
--- a/gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/PushOneCommit.java
+++ b/gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/PushOneCommit.java
@@ -18,6 +18,7 @@
 import static com.google.gerrit.acceptance.GitUtil.pushHead;
 
 import com.google.common.base.Strings;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Sets;
 import com.google.gerrit.common.Nullable;
@@ -44,6 +45,7 @@
 import org.eclipse.jgit.transport.RemoteRefUpdate.Status;
 
 import java.util.List;
+import java.util.Map;
 
 public class PushOneCommit {
   public static final String SUBJECT = "test commit";
@@ -91,6 +93,13 @@
         ReviewDb db,
         PersonIdent i,
         TestRepository<?> testRepo,
+        @Assisted String subject,
+        @Assisted Map<String, String> files);
+
+    PushOneCommit create(
+        ReviewDb db,
+        PersonIdent i,
+        TestRepository<?> testRepo,
         @Assisted("subject") String subject,
         @Assisted("fileName") String fileName,
         @Assisted("content") String content,
@@ -123,8 +132,7 @@
   private final TestRepository<?> testRepo;
 
   private final String subject;
-  private final String fileName;
-  private final String content;
+  private final Map<String, String> files;
   private String changeId;
   private Tag tag;
   private boolean force;
@@ -175,18 +183,43 @@
       @Assisted ReviewDb db,
       @Assisted PersonIdent i,
       @Assisted TestRepository<?> testRepo,
+      @Assisted String subject,
+      @Assisted Map<String, String> files) throws Exception {
+    this(notesFactory, approvalsUtil, queryProvider, db, i, testRepo,
+        subject, files, null);
+  }
+
+  @AssistedInject
+  PushOneCommit(ChangeNotes.Factory notesFactory,
+      ApprovalsUtil approvalsUtil,
+      Provider<InternalChangeQuery> queryProvider,
+      @Assisted ReviewDb db,
+      @Assisted PersonIdent i,
+      @Assisted TestRepository<?> testRepo,
       @Assisted("subject") String subject,
       @Assisted("fileName") String fileName,
       @Assisted("content") String content,
       @Nullable @Assisted("changeId") String changeId) throws Exception {
+    this(notesFactory, approvalsUtil, queryProvider, db, i, testRepo,
+        subject, ImmutableMap.of(fileName, content), changeId);
+  }
+
+  private PushOneCommit(ChangeNotes.Factory notesFactory,
+      ApprovalsUtil approvalsUtil,
+      Provider<InternalChangeQuery> queryProvider,
+      ReviewDb db,
+      PersonIdent i,
+      TestRepository<?> testRepo,
+      String subject,
+      Map<String, String> files,
+      String changeId) throws Exception {
     this.db = db;
     this.testRepo = testRepo;
     this.notesFactory = notesFactory;
     this.approvalsUtil = approvalsUtil;
     this.queryProvider = queryProvider;
     this.subject = subject;
-    this.fileName = fileName;
-    this.content = content;
+    this.files = files;
     this.changeId = changeId;
     if (changeId != null) {
       commitBuilder = testRepo.amendRef("HEAD")
@@ -207,12 +240,16 @@
   }
 
   public Result to(String ref) throws Exception {
-    commitBuilder.add(fileName, content);
+    for (Map.Entry<String, String> e : files.entrySet()) {
+      commitBuilder.add(e.getKey(), e.getValue());
+    }
     return execute(ref);
   }
 
   public Result rm(String ref) throws Exception {
-    commitBuilder.rm(fileName);
+    for (String fileName : files.keySet()) {
+      commitBuilder.rm(fileName);
+    }
     return execute(ref);
   }
 
diff --git a/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/revision/RevisionIT.java b/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/revision/RevisionIT.java
index 24cbac4..9533f3f 100644
--- a/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/revision/RevisionIT.java
+++ b/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/revision/RevisionIT.java
@@ -526,6 +526,35 @@
   }
 
   @Test
+  public void filesOnMergeCommitChange() throws Exception {
+    PushOneCommit.Result r = createMergeCommitChange("refs/for/master");
+
+    // list files against auto-merge
+    assertThat(gApi.changes()
+        .id(r.getChangeId())
+        .revision(r.getCommit().name())
+        .files()
+        .keySet()
+      ).containsExactly(Patch.COMMIT_MSG, "foo", "bar");
+
+    // list files against parent 1
+    assertThat(gApi.changes()
+        .id(r.getChangeId())
+        .revision(r.getCommit().name())
+        .files(1)
+        .keySet()
+      ).containsExactly(Patch.COMMIT_MSG, "bar");
+
+    // list files against parent 2
+    assertThat(gApi.changes()
+        .id(r.getChangeId())
+        .revision(r.getCommit().name())
+        .files(2)
+        .keySet()
+      ).containsExactly(Patch.COMMIT_MSG, "foo");
+  }
+
+  @Test
   public void diff() throws Exception {
     PushOneCommit.Result r = createChange();
     DiffInfo diff = gApi.changes()
@@ -538,6 +567,48 @@
   }
 
   @Test
+  public void diffOnMergeCommitChange() throws Exception {
+    PushOneCommit.Result r = createMergeCommitChange("refs/for/master");
+
+    DiffInfo diff;
+
+    // automerge
+    diff = gApi.changes()
+        .id(r.getChangeId())
+        .revision(r.getCommit().name())
+        .file("foo")
+        .diff();
+    assertThat(diff.metaA.lines).isEqualTo(5);
+    assertThat(diff.metaB.lines).isEqualTo(1);
+
+    diff = gApi.changes()
+        .id(r.getChangeId())
+        .revision(r.getCommit().name())
+        .file("bar")
+        .diff();
+    assertThat(diff.metaA.lines).isEqualTo(5);
+    assertThat(diff.metaB.lines).isEqualTo(1);
+
+    // parent 1
+    diff = gApi.changes()
+        .id(r.getChangeId())
+        .revision(r.getCommit().name())
+        .file("bar")
+        .diff(1);
+    assertThat(diff.metaA.lines).isEqualTo(1);
+    assertThat(diff.metaB.lines).isEqualTo(1);
+
+    // parent 2
+    diff = gApi.changes()
+        .id(r.getChangeId())
+        .revision(r.getCommit().name())
+        .file("foo")
+        .diff(2);
+    assertThat(diff.metaA.lines).isEqualTo(1);
+    assertThat(diff.metaB.lines).isEqualTo(1);
+  }
+
+  @Test
   public void content() throws Exception {
     PushOneCommit.Result r = createChange();
     BinaryResult bin = gApi.changes()
diff --git a/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/server/change/CommentsIT.java b/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/server/change/CommentsIT.java
index 96a672f..d9f1a5c 100644
--- a/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/server/change/CommentsIT.java
+++ b/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/server/change/CommentsIT.java
@@ -18,6 +18,7 @@
 import static com.google.gerrit.acceptance.PushOneCommit.FILE_NAME;
 import static com.google.gerrit.acceptance.PushOneCommit.SUBJECT;
 
+import com.google.common.base.Function;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Lists;
@@ -56,6 +57,7 @@
 
 @NoHttpd
 public class CommentsIT extends AbstractDaemonTest {
+
   @Inject
   private Provider<ChangesCollection> changes;
 
@@ -87,12 +89,35 @@
       PushOneCommit.Result r = createChange();
       String changeId = r.getChangeId();
       String revId = r.getCommit().getName();
-      DraftInput comment = newDraft("file1", Side.REVISION, line, "comment 1");
+      String path = "file1";
+      DraftInput comment = newDraft(path, Side.REVISION, line, "comment 1");
       addDraft(changeId, revId, comment);
       Map<String, List<CommentInfo>> result = getDraftComments(changeId, revId);
       assertThat(result).hasSize(1);
       CommentInfo actual = Iterables.getOnlyElement(result.get(comment.path));
-      assertCommentInfo(comment, actual);
+      assertThat(comment).isEqualTo(infoToDraft(path).apply(actual));
+    }
+  }
+
+  @Test
+  public void createDraftOnMergeCommitChange() throws Exception {
+    for (Integer line : lines) {
+      PushOneCommit.Result r = createMergeCommitChange("refs/for/master");
+      String changeId = r.getChangeId();
+      String revId = r.getCommit().getName();
+      String path = "file1";
+      DraftInput c1 = newDraft(path, Side.REVISION, line, "ps-1");
+      DraftInput c2 = newDraft(path, Side.PARENT, line, "auto-merge of ps-1");
+      DraftInput c3 = newDraftOnParent(path, 1, line, "parent-1 of ps-1");
+      DraftInput c4 = newDraftOnParent(path, 2, line, "parent-2 of ps-1");
+      addDraft(changeId, revId, c1);
+      addDraft(changeId, revId, c2);
+      addDraft(changeId, revId, c3);
+      addDraft(changeId, revId, c4);
+      Map<String, List<CommentInfo>> result = getDraftComments(changeId, revId);
+      assertThat(result).hasSize(1);
+      assertThat(Lists.transform(result.get(path), infoToDraft(path)))
+          .containsExactly(c1, c2, c3, c4);
     }
   }
 
@@ -114,8 +139,31 @@
       Map<String, List<CommentInfo>> result = getPublishedComments(changeId, revId);
       assertThat(result).isNotEmpty();
       CommentInfo actual = Iterables.getOnlyElement(result.get(comment.path));
-      assertCommentInfo(comment, actual);
-      assertCommentInfo(actual, getPublishedComment(changeId, revId, actual.id));
+      assertThat(comment).isEqualTo(infoToInput(file).apply(actual));
+      assertThat(comment).isEqualTo(infoToInput(file).apply(
+          getPublishedComment(changeId, revId, actual.id)));
+    }
+  }
+
+  @Test
+  public void postCommentOnMergeCommitChange() throws Exception {
+    for (Integer line : lines) {
+      final String file = "/COMMIT_MSG";
+      PushOneCommit.Result r = createMergeCommitChange("refs/for/master");
+      String changeId = r.getChangeId();
+      String revId = r.getCommit().getName();
+      ReviewInput input = new ReviewInput();
+      CommentInput c1 = newComment(file, Side.REVISION, line, "ps-1");
+      CommentInput c2 = newComment(file, Side.PARENT, line, "auto-merge of ps-1");
+      CommentInput c3 = newCommentOnParent(file, 1, line, "parent-1 of ps-1");
+      CommentInput c4 = newCommentOnParent(file, 2, line, "parent-2 of ps-1");
+      input.comments = new HashMap<>();
+      input.comments.put(file, ImmutableList.of(c1, c2, c3, c4));
+      revision(r).review(input);
+      Map<String, List<CommentInfo>> result = getPublishedComments(changeId, revId);
+      assertThat(result).isNotEmpty();
+      assertThat(Lists.transform(result.get(file), infoToInput(file)))
+          .containsExactly(c1, c2, c3, c4);
     }
   }
 
@@ -129,7 +177,7 @@
     String revId = r.getCommit().getName();
     assertThat(getPublishedComments(changeId, revId)).isEmpty();
 
-    List<Comment> expectedComments = new ArrayList<>();
+    List<CommentInput> expectedComments = new ArrayList<>();
     for (Integer line : lines) {
       ReviewInput input = new ReviewInput();
       CommentInput comment = newComment(file, Side.REVISION, line, "comment " + line);
@@ -142,10 +190,8 @@
     Map<String, List<CommentInfo>> result = getPublishedComments(changeId, revId);
     assertThat(result).isNotEmpty();
     List<CommentInfo> actualComments = result.get(file);
-    assertThat(actualComments).hasSize(expectedComments.size());
-    for (int i = 0; i < actualComments.size(); i++) {
-      assertCommentInfo(expectedComments.get(i), actualComments.get(i));
-    }
+    assertThat(Lists.transform(actualComments, infoToInput(file)))
+        .containsExactlyElementsIn(expectedComments);
   }
 
   @Test
@@ -155,17 +201,18 @@
       Timestamp origLastUpdated = r.getChange().change().getLastUpdatedOn();
       String changeId = r.getChangeId();
       String revId = r.getCommit().getName();
-      DraftInput comment = newDraft("file1", Side.REVISION, line, "comment 1");
+      String path = "file1";
+      DraftInput comment = newDraft(path, Side.REVISION, line, "comment 1");
       addDraft(changeId, revId, comment);
       Map<String, List<CommentInfo>> result = getDraftComments(changeId, revId);
       CommentInfo actual = Iterables.getOnlyElement(result.get(comment.path));
-      assertCommentInfo(comment, actual);
+      assertThat(comment).isEqualTo(infoToDraft(path).apply(actual));
       String uuid = actual.id;
       comment.message = "updated comment 1";
       updateDraft(changeId, revId, comment, uuid);
       result = getDraftComments(changeId, revId);
       actual = Iterables.getOnlyElement(result.get(comment.path));
-      assertCommentInfo(comment, actual);
+      assertThat(comment).isEqualTo(infoToDraft(path).apply(actual));
 
       // Posting a draft comment doesn't cause lastUpdatedOn to change.
       assertThat(r.getChange().change().getLastUpdatedOn())
@@ -181,7 +228,7 @@
     String revId = r.getCommit().getName();
     assertThat(getDraftComments(changeId, revId)).isEmpty();
 
-    List<Comment> expectedDrafts = new ArrayList<>();
+    List<DraftInput> expectedDrafts = new ArrayList<>();
     for (Integer line : lines) {
       DraftInput comment = newDraft(file, Side.REVISION, line, "comment " + line);
       expectedDrafts.add(comment);
@@ -191,10 +238,8 @@
     Map<String, List<CommentInfo>> result = getDraftComments(changeId, revId);
     assertThat(result).isNotEmpty();
     List<CommentInfo> actualComments = result.get(file);
-    assertThat(actualComments).hasSize(expectedDrafts.size());
-    for (int i = 0; i < actualComments.size(); i++) {
-      assertCommentInfo(expectedDrafts.get(i), actualComments.get(i));
-    }
+    assertThat(Lists.transform(actualComments, infoToDraft(file)))
+        .containsExactlyElementsIn(expectedDrafts);
   }
 
   @Test
@@ -203,11 +248,12 @@
       PushOneCommit.Result r = createChange();
       String changeId = r.getChangeId();
       String revId = r.getCommit().getName();
+      String path = "file1";
       DraftInput comment = newDraft(
-          "file1", Side.REVISION, line, "comment 1");
+          path, Side.REVISION, line, "comment 1");
       CommentInfo returned = addDraft(changeId, revId, comment);
       CommentInfo actual = getDraftComment(changeId, revId, returned.id);
-      assertCommentInfo(comment, actual);
+      assertThat(comment).isEqualTo(infoToDraft(path).apply(actual));
     }
   }
 
@@ -257,7 +303,9 @@
       Map<String, List<CommentInfo>> result = getPublishedComments(changeId, revId);
       assertThat(result).isNotEmpty();
       CommentInfo actual = Iterables.getOnlyElement(result.get(comment.path));
-      assertCommentInfo(comment, actual);
+      CommentInput ci = infoToInput(file).apply(actual);
+      ci.updated = comment.updated;
+      assertThat(comment).isEqualTo(ci);
       assertThat(actual.updated)
           .isEqualTo(gApi.changes().id(r.getChangeId()).info().created);
 
@@ -597,45 +645,35 @@
     return gApi.changes().id(changeId).revision(revId).draft(uuid).get();
   }
 
-  private static void assertCommentInfo(Comment expected, CommentInfo actual) {
-    assertThat(actual.line).isEqualTo(expected.line);
-    assertThat(actual.message).isEqualTo(expected.message);
-    assertThat(actual.inReplyTo).isEqualTo(expected.inReplyTo);
-    assertCommentRange(expected.range, actual.range);
-    if (actual.side == null && expected.side != null) {
-      assertThat(Side.REVISION).isEqualTo(expected.side);
-    }
-  }
-
-  private static void assertCommentRange(Comment.Range expected,
-      Comment.Range actual) {
-    if (expected == null) {
-      assertThat(actual).isNull();
-    } else {
-      assertThat(actual).isNotNull();
-      assertThat(actual.startLine).isEqualTo(expected.startLine);
-      assertThat(actual.startCharacter).isEqualTo(expected.startCharacter);
-      assertThat(actual.endLine).isEqualTo(expected.endLine);
-      assertThat(actual.endCharacter).isEqualTo(expected.endCharacter);
-    }
-  }
-
   private static CommentInput newComment(String path, Side side, int line,
       String message) {
     CommentInput c = new CommentInput();
-    return populate(c, path, side, line, message);
+    return populate(c, path, side, null, line, message);
+  }
+
+  private static CommentInput newCommentOnParent(String path, int parent,
+      int line, String message) {
+    CommentInput c = new CommentInput();
+    return populate(c, path, Side.PARENT, Integer.valueOf(parent), line, message);
   }
 
   private DraftInput newDraft(String path, Side side, int line,
       String message) {
     DraftInput d = new DraftInput();
-    return populate(d, path, side, line, message);
+    return populate(d, path, side, null, line, message);
+  }
+
+  private DraftInput newDraftOnParent(String path, int parent, int line,
+      String message) {
+    DraftInput d = new DraftInput();
+    return populate(d, path, Side.PARENT, Integer.valueOf(parent), line, message);
   }
 
   private static <C extends Comment> C populate(C c, String path, Side side,
-      int line, String message) {
+      Integer parent, int line, String message) {
     c.path = path;
     c.side = side;
+    c.parent = parent;
     c.line = line != 0 ? line : null;
     c.message = message;
     if (line != 0) {
@@ -648,4 +686,38 @@
     }
     return c;
   }
+
+  private static Function<CommentInfo, CommentInput> infoToInput(
+      final String path) {
+    return new Function<CommentInfo, CommentInput>() {
+      @Override
+      public CommentInput apply(CommentInfo info) {
+        CommentInput ci = new CommentInput();
+        ci.path = path;
+        copy(info, ci);
+        return ci;
+      }
+    };
+  }
+
+  private static Function<CommentInfo, DraftInput> infoToDraft(
+      final String path) {
+    return new Function<CommentInfo, DraftInput>() {
+      @Override
+      public DraftInput apply(CommentInfo info) {
+        DraftInput di = new DraftInput();
+        di.path = path;
+        copy(info, di);
+        return di;
+      }
+    };
+  }
+
+  private static void copy(Comment from, Comment to) {
+    to.side = from.side == null ? Side.REVISION : from.side;
+    to.parent = from.parent;
+    to.line = from.line;
+    to.message = from.message;
+    to.range = from.range;
+  }
 }
diff --git a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/DraftInput.java b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/DraftInput.java
index 8b626b7..9d94f50 100644
--- a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/DraftInput.java
+++ b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/DraftInput.java
@@ -16,6 +16,22 @@
 
 import com.google.gerrit.extensions.client.Comment;
 
+import java.util.Objects;
+
 public class DraftInput extends Comment {
   public String tag;
+
+  @Override
+  public boolean equals(Object o) {
+    if (super.equals(o)) {
+      DraftInput di = (DraftInput) o;
+      return Objects.equals(tag, di.tag);
+    }
+    return false;
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(super.hashCode(), tag);
+  }
 }
diff --git a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/FileApi.java b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/FileApi.java
index 3641ac5..2536c46 100644
--- a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/FileApi.java
+++ b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/FileApi.java
@@ -35,6 +35,11 @@
   DiffInfo diff(String base) throws RestApiException;
 
   /**
+   * @param parent 1-based parent number to diff against
+   */
+  DiffInfo diff(int parent) throws RestApiException;
+
+  /**
    * Creates a request to retrieve the diff. On the returned request formatting
    * options for the diff can be set.
    */
@@ -106,6 +111,11 @@
     }
 
     @Override
+    public DiffInfo diff(int parent) throws RestApiException {
+      throw new NotImplementedException();
+    }
+
+    @Override
     public DiffRequest diffRequest() throws RestApiException {
       throw new NotImplementedException();
     }
diff --git a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/RevisionApi.java b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/RevisionApi.java
index b23c7f9..2731476 100644
--- a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/RevisionApi.java
+++ b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/RevisionApi.java
@@ -46,6 +46,7 @@
 
   Map<String, FileInfo> files() throws RestApiException;
   Map<String, FileInfo> files(String base) throws RestApiException;
+  Map<String, FileInfo> files(int parentNum) throws RestApiException;
   FileApi file(String path);
   MergeableInfo mergeable() throws RestApiException;
   MergeableInfo mergeableOtherBranches() throws RestApiException;
@@ -147,6 +148,11 @@
     }
 
     @Override
+    public Map<String, FileInfo> files(int parentNum) throws RestApiException {
+      throw new NotImplementedException();
+    }
+
+    @Override
     public Map<String, FileInfo> files() throws RestApiException {
       throw new NotImplementedException();
     }
diff --git a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/client/Comment.java b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/client/Comment.java
index b9863d7..7c8a3e8 100644
--- a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/client/Comment.java
+++ b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/client/Comment.java
@@ -15,6 +15,7 @@
 package com.google.gerrit.extensions.client;
 
 import java.sql.Timestamp;
+import java.util.Objects;
 
 public abstract class Comment {
   /**
@@ -27,6 +28,7 @@
   public String id;
   public String path;
   public Side side;
+  public Integer parent;
   public Integer line;
   public Range range;
   public String inReplyTo;
@@ -38,5 +40,49 @@
     public int startCharacter;
     public int endLine;
     public int endCharacter;
+
+    @Override
+    public boolean equals(Object o) {
+      if (o instanceof Range) {
+        Range r = (Range) o;
+        return Objects.equals(startLine, r.startLine)
+            && Objects.equals(startCharacter, r.startCharacter)
+            && Objects.equals(endLine, r.endLine)
+            && Objects.equals(endCharacter, r.endCharacter);
+      }
+      return false;
+    }
+
+    @Override
+    public int hashCode() {
+      return Objects.hash(startLine, startCharacter, endLine, endCharacter);
+    }
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o != null && getClass() == o.getClass()) {
+      Comment c = (Comment) o;
+      return Objects.equals(patchSet, c.patchSet)
+          && Objects.equals(id, c.id)
+          && Objects.equals(path, c.path)
+          && Objects.equals(side, c.side)
+          && Objects.equals(parent, c.parent)
+          && Objects.equals(line, c.line)
+          && Objects.equals(range, c.range)
+          && Objects.equals(inReplyTo, c.inReplyTo)
+          && Objects.equals(updated, c.updated)
+          && Objects.equals(message, c.message);
+    }
+    return false;
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(patchSet, id, path, side, parent, line, range,
+        inReplyTo, updated, message);
   }
 }
diff --git a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/client/Side.java b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/client/Side.java
index 3485b8b..e077df2 100644
--- a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/client/Side.java
+++ b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/client/Side.java
@@ -19,11 +19,10 @@
   REVISION;
 
   public static Side fromShort(short s) {
-    switch (s) {
-      case 0:
-        return PARENT;
-      case 1:
-        return REVISION;
+    if (s <= 0) {
+      return PARENT;
+    } else if (s == 1) {
+      return REVISION;
     }
     return null;
   }
diff --git a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/common/CommentInfo.java b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/common/CommentInfo.java
index b7535e1..166aaa2 100644
--- a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/common/CommentInfo.java
+++ b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/common/CommentInfo.java
@@ -16,7 +16,24 @@
 
 import com.google.gerrit.extensions.client.Comment;
 
+import java.util.Objects;
+
 public class CommentInfo extends Comment {
   public AccountInfo author;
   public String tag;
+
+  @Override
+  public boolean equals(Object o) {
+    if (super.equals(o)) {
+      CommentInfo ci = (CommentInfo) o;
+      return Objects.equals(author, ci.author)
+          && Objects.equals(tag, ci.tag);
+    }
+    return false;
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(super.hashCode(), author, tag);
+  }
 }
diff --git a/gerrit-gwtui-common/src/main/java/com/google/gerrit/client/info/ChangeInfo.java b/gerrit-gwtui-common/src/main/java/com/google/gerrit/client/info/ChangeInfo.java
index 053b8c5..9eea93e 100644
--- a/gerrit-gwtui-common/src/main/java/com/google/gerrit/client/info/ChangeInfo.java
+++ b/gerrit-gwtui-common/src/main/java/com/google/gerrit/client/info/ChangeInfo.java
@@ -333,12 +333,22 @@
       revisionInfo.takeFromEdit(edit);
       return revisionInfo;
     }
+    public static RevisionInfo forParent(int number, CommitInfo commit) {
+      RevisionInfo revisionInfo = createObject().cast();
+      revisionInfo.takeFromParent(number, commit);
+      return revisionInfo;
+    }
     private native void takeFromEdit(EditInfo edit) /*-{
       this._number = 0;
       this.name = edit.name;
       this.commit = edit.commit;
       this.edit_base = edit.base_revision;
     }-*/;
+    private native void takeFromParent(int number, CommitInfo commit) /*-{
+      this._number = number;
+      this.commit = commit;
+      this.name = this._number;
+    }-*/;
     public final native int _number() /*-{ return this._number; }-*/;
     public final native String name() /*-{ return this.name; }-*/;
     public final native boolean draft() /*-{ return this.draft || false; }-*/;
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/change/ChangeScreen.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/change/ChangeScreen.java
index 340460f..8da949c 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/change/ChangeScreen.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/change/ChangeScreen.java
@@ -981,24 +981,25 @@
       final List<NativeMap<JsArray<CommentInfo>>> comments,
       final List<NativeMap<JsArray<CommentInfo>>> drafts) {
     DiffApi.list(changeId.get(),
-      base != null ? base.name() : null,
-      rev.name(),
-      group.add(new AsyncCallback<NativeMap<FileInfo>>() {
-        @Override
-        public void onSuccess(NativeMap<FileInfo> m) {
-          files.set(
-              base != null ? new PatchSet.Id(changeId, base._number()) : null,
-              new PatchSet.Id(changeId, rev._number()),
-              style, reply, fileTableMode, edit != null);
-          files.setValue(m, myLastReply,
-              comments != null ? comments.get(0) : null,
-              drafts != null ? drafts.get(0) : null);
-        }
+        rev.name(),
+        base,
+        group.add(
+            new AsyncCallback<NativeMap<FileInfo>>() {
+              @Override
+              public void onSuccess(NativeMap<FileInfo> m) {
+                files.set(
+                    base != null ? new PatchSet.Id(changeId, base._number()) : null,
+                    new PatchSet.Id(changeId, rev._number()),
+                    style, reply, fileTableMode, edit != null);
+                files.setValue(m, myLastReply,
+                    comments != null ? comments.get(0) : null,
+                    drafts != null ? drafts.get(0) : null);
+              }
 
-        @Override
-        public void onFailure(Throwable caught) {
-        }
-      }));
+              @Override
+              public void onFailure(Throwable caught) {
+              }
+            }));
   }
 
   private List<NativeMap<JsArray<CommentInfo>>> loadComments(
@@ -1117,7 +1118,6 @@
   }
 
   /**
-   *
    * Resolve a revision or patch set id string to RevisionInfo.
    * When this view is created from the changes table, revision
    * is passed as a real revision.
@@ -1131,8 +1131,17 @@
    */
   private RevisionInfo resolveRevisionOrPatchSetId(ChangeInfo info,
       String revOrId, String defaultValue) {
+    int parentNum;
     if (revOrId == null) {
       revOrId = defaultValue;
+    } else if ((parentNum = toParentNum(revOrId)) > 0) {
+      CommitInfo commitInfo = info.revision(revision).commit();
+      if (commitInfo != null) {
+        JsArray<CommitInfo> parents = commitInfo.parents();
+        if (parents.length() >= parentNum) {
+          return RevisionInfo.forParent(-parentNum, parents.get(parentNum - 1));
+        }
+      }
     } else if (!info.revisions().containsKey(revOrId)) {
       JsArray<RevisionInfo> list = info.revisions().values();
       for (int i = 0; i < list.length(); i++) {
@@ -1389,9 +1398,20 @@
 
     RevisionInfo rev = info.revisions().get(revision);
     JsArray<CommitInfo> parents = rev.commit().parents();
-    diffBase.addItem(
-      parents.length() > 1 ? Util.C.autoMerge() : Util.C.baseDiffItem(),
-      "");
+    if (parents.length() > 1) {
+      diffBase.addItem(Util.C.autoMerge(), "");
+      for (int i = 0; i < parents.length(); i++) {
+        int parentNum = i + 1;
+        diffBase.addItem(Util.M.diffBaseParent(parentNum),
+            String.valueOf(-parentNum));
+      }
+      int parentNum = toParentNum(base);
+      if (parentNum > 0) {
+        selectedIdx = list.length() + parentNum;
+      }
+    } else {
+      diffBase.addItem(Util.C.baseDiffItem(), "");
+    }
 
     diffBase.setSelectedIndex(selectedIdx);
   }
@@ -1443,4 +1463,22 @@
   private static String normalize(String r) {
     return r != null && !r.isEmpty() ? r : null;
   }
+
+  /**
+   * @param parentToken
+   * @return 1-based parentNum if parentToken is a String which can be parsed as
+   *     a negative integer i.e. "-1", "-2", etc. If parentToken cannot be
+   *     parsed as a negative integer, return zero.
+   */
+  private static int toParentNum(String parentToken) {
+    try {
+      int n = Integer.parseInt(parentToken);
+      if (n < 0) {
+        return -n;
+      }
+      return 0;
+    } catch (NumberFormatException e) {
+      return 0;
+    }
+  }
 }
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/change/FileTable.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/change/FileTable.java
index 60d66e0..f0a7ce3 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/change/FileTable.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/change/FileTable.java
@@ -36,6 +36,7 @@
 import com.google.gerrit.client.ui.NavigationTable;
 import com.google.gerrit.common.Nullable;
 import com.google.gerrit.common.PageLinks;
+import com.google.gerrit.extensions.client.Side;
 import com.google.gerrit.reviewdb.client.Patch;
 import com.google.gerrit.reviewdb.client.Patch.ChangeType;
 import com.google.gerrit.reviewdb.client.PatchSet;
@@ -712,8 +713,8 @@
     }
 
     private void columnComments(SafeHtmlBuilder sb, FileInfo info) {
-      JsArray<CommentInfo> cList = get(info.path(), comments);
-      JsArray<CommentInfo> dList = get(info.path(), drafts);
+      JsArray<CommentInfo> cList = filterForParent(get(info.path(), comments));
+      JsArray<CommentInfo> dList = filterForParent(get(info.path(), drafts));
 
       sb.openTd().setStyleName(R.css().draftColumn());
       if (dList.length() > 0) {
@@ -747,6 +748,20 @@
       sb.closeTd();
     }
 
+    private JsArray<CommentInfo> filterForParent(JsArray<CommentInfo> list) {
+      JsArray<CommentInfo> result = JsArray.createArray().cast();
+      for (CommentInfo c : Natives.asList(list)) {
+        if (c.side() == Side.REVISION) {
+          result.push(c);
+        } else if (base == null && !c.hasParent()) {
+          result.push(c);
+        } else if (base != null && c.parent() == -base.get()) {
+          result.push(c);
+        }
+      }
+      return result;
+    }
+
     private JsArray<CommentInfo> get(String p, NativeMap<JsArray<CommentInfo>> m) {
       JsArray<CommentInfo> r = null;
       if (m != null) {
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeMessages.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeMessages.java
index c5397ee..b192bd5 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeMessages.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeMessages.java
@@ -42,4 +42,6 @@
   String changeQueryPageTitle(String query);
 
   String insertionsAndDeletions(int insertions, int deletions);
+
+  String diffBaseParent(int parentNum);
 }
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeMessages.properties b/gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeMessages.properties
index f0d7e59..2b68492 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeMessages.properties
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeMessages.properties
@@ -23,3 +23,5 @@
 changeQueryPageTitle = Search for {0}
 
 insertionsAndDeletions = +{0}, -{1}
+
+diffBaseParent = Parent {0}
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/CommentInfo.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/CommentInfo.java
index 8e73f73..d42c344 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/CommentInfo.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/CommentInfo.java
@@ -25,9 +25,15 @@
 public class CommentInfo extends JavaScriptObject {
   public static CommentInfo create(String path, Side side,
       int line, CommentRange range) {
+    return create(path, side, 0, line, range);
+  }
+
+  public static CommentInfo create(String path, Side side, int parent,
+      int line, CommentRange range) {
     CommentInfo n = createObject().cast();
     n.path(path);
     n.side(side);
+    n.parent(parent);
     if (range != null) {
       n.line(range.endLine());
       n.range(range);
@@ -41,6 +47,7 @@
     CommentInfo n = createObject().cast();
     n.path(r.path());
     n.side(r.side());
+    n.parent(r.parent());
     n.inReplyTo(r.id());
     if (r.hasRange()) {
       n.line(r.range().endLine());
@@ -55,6 +62,7 @@
     CommentInfo n = createObject().cast();
     n.path(s.path());
     n.side(s.side());
+    n.parent(s.parent());
     n.id(s.id());
     n.inReplyTo(s.inReplyTo());
     n.message(s.message());
@@ -78,6 +86,8 @@
     sideRaw(side.toString());
   }
   private native void sideRaw(String s) /*-{ this.side = s }-*/;
+  public final native void parent(int n) /*-{ this.parent = n }-*/;
+  public final native boolean hasParent() /*-{ return this.hasOwnProperty('parent') }-*/;
 
   public final native String path() /*-{ return this.path }-*/;
   public final native String id() /*-{ return this.id }-*/;
@@ -91,6 +101,7 @@
         : Side.REVISION;
   }
   private native String sideRaw() /*-{ return this.side }-*/;
+  public final native int parent() /*-{ return this.parent }-*/;
 
   public final Timestamp updated() {
     Timestamp r = updatedTimestamp();
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentManager.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentManager.java
index a26b1ce..2f3ead3 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentManager.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentManager.java
@@ -129,16 +129,29 @@
   }
 
   Side getStoredSideFromDisplaySide(DisplaySide side) {
-    return side == DisplaySide.A && base == null ? Side.PARENT : Side.REVISION;
+    if (side == DisplaySide.A && (base == null || base.get() < 0)) {
+      return Side.PARENT;
+    }
+    return Side.REVISION;
+  }
+
+  int getParentNumFromDisplaySide(DisplaySide side) {
+    if (side == DisplaySide.A && base != null && base.get() < 0) {
+      return -base.get();
+    }
+    return 0;
   }
 
   PatchSet.Id getPatchSetIdFromSide(DisplaySide side) {
-    return side == DisplaySide.A && base != null ? base : revision;
+    if (side == DisplaySide.A && base != null && base.get() >= 0) {
+      return base;
+    }
+    return revision;
   }
 
   DisplaySide displaySide(CommentInfo info, DisplaySide forSide) {
     if (info.side() == Side.PARENT) {
-      return base == null ? DisplaySide.A : null;
+      return (base == null || base.get() < 0) ? DisplaySide.A : null;
     }
     return forSide;
   }
@@ -179,6 +192,7 @@
       addDraftBox(side, CommentInfo.create(
           getPath(),
           getStoredSideFromDisplaySide(side),
+          getParentNumFromDisplaySide(side),
           line,
           null)).setEdit(true);
     }
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentsCollections.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentsCollections.java
index 83f74a3..ce1d294 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentsCollections.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentsCollections.java
@@ -20,6 +20,7 @@
 import com.google.gerrit.client.rpc.CallbackGroup;
 import com.google.gerrit.client.rpc.NativeMap;
 import com.google.gerrit.client.rpc.Natives;
+import com.google.gerrit.extensions.client.Side;
 import com.google.gerrit.reviewdb.client.PatchSet;
 import com.google.gwt.core.client.JsArray;
 import com.google.gwt.user.client.rpc.AsyncCallback;
@@ -46,13 +47,13 @@
   }
 
   void load(CallbackGroup group) {
-    if (base != null) {
+    if (base != null && base.get() > 0) {
       CommentApi.comments(base, group.add(publishedBase()));
     }
     CommentApi.comments(revision, group.add(publishedRevision()));
 
     if (Gerrit.isSignedIn()) {
-      if (base != null) {
+      if (base != null && base.get() > 0) {
         CommentApi.drafts(base, group.add(draftsBase()));
       }
       CommentApi.drafts(revision, group.add(draftsRevision()));
@@ -60,7 +61,7 @@
   }
 
   boolean hasCommentForPath(String filePath) {
-    if (base != null) {
+    if (base != null && base.get() > 0) {
       JsArray<CommentInfo> forBase = publishedBaseAll.get(filePath);
       if (forBase != null && forBase.length() > 0) {
         return true;
@@ -91,6 +92,9 @@
     return new AsyncCallback<NativeMap<JsArray<CommentInfo>>>() {
       @Override
       public void onSuccess(NativeMap<JsArray<CommentInfo>> result) {
+        for (String k : result.keySet()) {
+          result.put(k, filterForParent(result.get(k)));
+        }
         publishedRevisionAll = result;
         publishedRevision = sort(result.get(path));
       }
@@ -101,6 +105,20 @@
     };
   }
 
+    private JsArray<CommentInfo> filterForParent(JsArray<CommentInfo> list) {
+      JsArray<CommentInfo> result = JsArray.createArray().cast();
+      for (CommentInfo c : Natives.asList(list)) {
+        if (c.side() == Side.REVISION) {
+          result.push(c);
+        } else if (base == null && !c.hasParent()) {
+          result.push(c);
+        } else if (base != null && c.parent() == -base.get()) {
+          result.push(c);
+        }
+      }
+      return result;
+    }
+
   private AsyncCallback<NativeMap<JsArray<CommentInfo>>> draftsBase() {
     return new AsyncCallback<NativeMap<JsArray<CommentInfo>>>() {
       @Override
@@ -118,6 +136,9 @@
     return new AsyncCallback<NativeMap<JsArray<CommentInfo>>>() {
       @Override
       public void onSuccess(NativeMap<JsArray<CommentInfo>> result) {
+        for (String k : result.keySet()) {
+          result.put(k, filterForParent(result.get(k)));
+        }
         draftsRevision = sort(result.get(path));
       }
 
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffApi.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffApi.java
index bc5a305..e3720cc 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffApi.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffApi.java
@@ -17,6 +17,7 @@
 import static com.google.gerrit.extensions.client.DiffPreferencesInfo.Whitespace.IGNORE_ALL;
 
 import com.google.gerrit.client.changes.ChangeApi;
+import com.google.gerrit.client.info.ChangeInfo.RevisionInfo;
 import com.google.gerrit.client.info.FileInfo;
 import com.google.gerrit.client.rpc.NativeMap;
 import com.google.gerrit.client.rpc.RestApi;
@@ -25,11 +26,15 @@
 import com.google.gwt.user.client.rpc.AsyncCallback;
 
 public class DiffApi {
-  public static void list(int id, String base, String revision,
+  public static void list(int id, String revision, RevisionInfo base,
       AsyncCallback<NativeMap<FileInfo>> cb) {
     RestApi api = ChangeApi.revision(id, revision).view("files");
     if (base != null) {
-      api.addParameter("base", base);
+      if (base._number() < 0) {
+        api.addParameter("parent", -base._number());
+      } else {
+        api.addParameter("base", base.name());
+      }
     }
     api.get(NativeMap.copyKeysIntoChildren("path", cb));
   }
@@ -38,7 +43,11 @@
       AsyncCallback<NativeMap<FileInfo>> cb) {
     RestApi api = ChangeApi.revision(id).view("files");
     if (base != null) {
-      api.addParameter("base", base.get());
+      if (base.get() < 0) {
+        api.addParameter("parent", -base.get());
+      } else {
+        api.addParameter("base", base.get());
+      }
     }
     api.get(NativeMap.copyKeysIntoChildren("path", cb));
   }
@@ -57,7 +66,11 @@
 
   public DiffApi base(PatchSet.Id id) {
     if (id != null) {
-      call.addParameter("base", id.get());
+      if (id.get() < 0) {
+        call.addParameter("parent", -id.get());
+      } else {
+        call.addParameter("base", id.get());
+      }
     }
     return this;
   }
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffScreen.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffScreen.java
index 2264871..8935e36 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffScreen.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffScreen.java
@@ -27,6 +27,7 @@
 import com.google.gerrit.client.diff.DiffInfo.FileMeta;
 import com.google.gerrit.client.diff.LineMapper.LineOnOtherInfo;
 import com.google.gerrit.client.info.ChangeInfo;
+import com.google.gerrit.client.info.ChangeInfo.CommitInfo;
 import com.google.gerrit.client.info.ChangeInfo.EditInfo;
 import com.google.gerrit.client.info.ChangeInfo.RevisionInfo;
 import com.google.gerrit.client.info.FileInfo;
@@ -116,6 +117,7 @@
   private List<HandlerRegistration> handlers;
   private PreferencesAction prefsAction;
   private int reloadVersionId;
+  private int parents;
 
   @UiField(provided = true)
   Header header;
@@ -213,6 +215,8 @@
         new CommentsCollections(base, revision, path);
     comments.load(group2);
 
+    countParents(group2);
+
     RestApi call = ChangeApi.detail(changeId.get());
     ChangeList.addOptions(call, EnumSet.of(
         ListChangesOption.ALL_REVISIONS));
@@ -231,7 +235,7 @@
             revision.get() == info.revision(currentRevision)._number();
         JsArray<RevisionInfo> list = info.revisions().values();
         RevisionInfo.sortRevisionInfoByNumber(list);
-        getDiffTable().set(prefs, list, diff, edit != null, current,
+        getDiffTable().set(prefs, list, parents, diff, edit != null, current,
             changeStatus.isOpen(), diff.binary());
         header.setChangeInfo(info);
       }
@@ -245,6 +249,22 @@
         getScreenLoadCallback(comments)));
   }
 
+  private void countParents(CallbackGroup cbg) {
+    ChangeApi.revision(changeId.get(), revision.getId())
+        .view("commit")
+        .get(cbg.add(new AsyncCallback<CommitInfo>() {
+          @Override
+          public void onSuccess(CommitInfo info) {
+            parents = info.parents().length();
+          }
+
+          @Override
+          public void onFailure(Throwable caught) {
+            parents = 0;
+          }
+        }));
+  }
+
   @Override
   public void onShowView() {
     super.onShowView();
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.java
index 4374986..392ad2f 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.java
@@ -108,12 +108,12 @@
     patchSetSelectBoxB.setUpBlame(cm, false, rev, path);
   }
 
-  void set(DiffPreferences prefs, JsArray<RevisionInfo> list, DiffInfo info,
+  void set(DiffPreferences prefs, JsArray<RevisionInfo> list, int parents, DiffInfo info,
       boolean editExists, boolean current, boolean open, boolean binary) {
     this.changeType = info.changeType();
-    patchSetSelectBoxA.setUpPatchSetNav(list, info.metaA(), editExists,
+    patchSetSelectBoxA.setUpPatchSetNav(list, parents, info.metaA(), editExists,
         current, open, binary);
-    patchSetSelectBoxB.setUpPatchSetNav(list, info.metaB(), editExists,
+    patchSetSelectBoxB.setUpPatchSetNav(list, parents, info.metaB(), editExists,
         current, open, binary);
 
     JsArrayString hdr = info.diffHeader();
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/PatchSetSelectBox.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/PatchSetSelectBox.java
index 39b85cf..bc37abb 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/PatchSetSelectBox.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/PatchSetSelectBox.java
@@ -18,6 +18,7 @@
 import com.google.gerrit.client.Gerrit;
 import com.google.gerrit.client.blame.BlameInfo;
 import com.google.gerrit.client.changes.ChangeApi;
+import com.google.gerrit.client.changes.Util;
 import com.google.gerrit.client.info.ChangeInfo.RevisionInfo;
 import com.google.gerrit.client.info.WebLinkInfo;
 import com.google.gerrit.client.patches.PatchUtil;
@@ -87,13 +88,29 @@
     this.path = path;
   }
 
-  void setUpPatchSetNav(JsArray<RevisionInfo> list, DiffInfo.FileMeta meta,
+  void setUpPatchSetNav(JsArray<RevisionInfo> list, int parents, DiffInfo.FileMeta meta,
       boolean editExists, boolean current, boolean open, boolean binary) {
-    InlineHyperlink baseLink = null;
     InlineHyperlink selectedLink = null;
     if (sideA) {
-      baseLink = createLink(PatchUtil.C.patchBase(), null);
-      linkPanel.add(baseLink);
+      if (parents <= 1) {
+        InlineHyperlink link = createLink(PatchUtil.C.patchBase(), null);
+        linkPanel.add(link);
+        selectedLink = link;
+      } else {
+        for (int i = parents; i > 0; i--) {
+          PatchSet.Id id = new PatchSet.Id(changeId, -i);
+          InlineHyperlink link = createLink(Util.M.diffBaseParent(i), id);
+          linkPanel.add(link);
+          if (revision != null && id.equals(revision)) {
+            selectedLink = link;
+          }
+        }
+        InlineHyperlink link = createLink(Util.C.autoMerge(), null);
+        linkPanel.add(link);
+        if (selectedLink == null) {
+          selectedLink = link;
+        }
+      }
     }
     for (int i = 0; i < list.length(); i++) {
       RevisionInfo r = list.get(i);
@@ -106,8 +123,6 @@
     }
     if (selectedLink != null) {
       selectedLink.setStyleName(style.selected());
-    } else if (sideA) {
-      baseLink.setStyleName(style.selected());
     }
 
     if (meta == null) {
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/SideBySideCommentManager.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/SideBySideCommentManager.java
index 2b83b71..bcb7dac 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/SideBySideCommentManager.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/SideBySideCommentManager.java
@@ -84,6 +84,7 @@
       addDraftBox(cm.side(), CommentInfo.create(
               getPath(),
               getStoredSideFromDisplaySide(cm.side()),
+              getParentNumFromDisplaySide(cm.side()),
               line,
               CommentRange.create(fromTo))).setEdit(true);
       cm.setCursor(fromTo.to());
diff --git a/gerrit-server/src/main/java/com/google/gerrit/rules/StoredValues.java b/gerrit-server/src/main/java/com/google/gerrit/rules/StoredValues.java
index 9034e47..c493ccd 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/rules/StoredValues.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/rules/StoredValues.java
@@ -100,10 +100,9 @@
       PatchListCache plCache = env.getArgs().getPatchListCache();
       Change change = getChange(engine);
       Project.NameKey project = change.getProject();
-      ObjectId a = null;
       ObjectId b = ObjectId.fromString(ps.getRevision().get());
       Whitespace ws = Whitespace.IGNORE_NONE;
-      PatchListKey plKey = new PatchListKey(a, b, ws);
+      PatchListKey plKey = PatchListKey.againstDefaultBase(b, ws);
       PatchList patchList;
       try {
         patchList = plCache.get(plKey, project);
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/PatchLineCommentsUtil.java b/gerrit-server/src/main/java/com/google/gerrit/server/PatchLineCommentsUtil.java
index d990115..603f528 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/PatchLineCommentsUtil.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/PatchLineCommentsUtil.java
@@ -365,9 +365,17 @@
         "cannot set RevId for patch set %s on comment %s", ps.getId(), c);
     if (c.getRevId() == null) {
       try {
-        c.setRevId(Side.fromShort(c.getSide()) == Side.PARENT
-            ? new RevId(ObjectId.toString(cache.getOldId(change, ps)))
-            : ps.getRevision());
+        if (Side.fromShort(c.getSide()) == Side.PARENT) {
+          if (c.getSide() < 0) {
+            c.setRevId(new RevId(ObjectId.toString(
+                cache.getOldId(change, ps, -c.getSide()))));
+          } else {
+            c.setRevId(new RevId(ObjectId.toString(
+                cache.getOldId(change, ps, null))));
+          }
+        } else {
+          c.setRevId(ps.getRevision());
+        }
       } catch (PatchListNotAvailableException e) {
         throw new OrmException(e);
       }
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/api/changes/FileApiImpl.java b/gerrit-server/src/main/java/com/google/gerrit/server/api/changes/FileApiImpl.java
index 3d40373..e6ca18df 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/api/changes/FileApiImpl.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/api/changes/FileApiImpl.java
@@ -75,6 +75,15 @@
   }
 
   @Override
+  public DiffInfo diff(int parent) throws RestApiException {
+    try {
+      return getDiff.setParent(parent).apply(file).value();
+    } catch (OrmException | InvalidChangeOperationException | IOException e) {
+      throw new RestApiException("Cannot retrieve diff", e);
+    }
+  }
+
+  @Override
   public DiffRequest diffRequest() {
     return new DiffRequest() {
       @Override
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/api/changes/RevisionApiImpl.java b/gerrit-server/src/main/java/com/google/gerrit/server/api/changes/RevisionApiImpl.java
index 4c7adea..f8ef00e 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/api/changes/RevisionApiImpl.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/api/changes/RevisionApiImpl.java
@@ -309,6 +309,17 @@
     }
   }
 
+  @SuppressWarnings("unchecked")
+  @Override
+  public Map<String, FileInfo> files(int parentNum) throws RestApiException {
+    try {
+      return (Map<String, FileInfo>) listFiles.setParent(parentNum)
+          .apply(revision).value();
+    } catch (OrmException | IOException e) {
+      throw new RestApiException("Cannot retrieve files", e);
+    }
+  }
+
   @Override
   public FileApi file(String path) {
     return fileApi.create(files.parse(revision,
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/CommentJson.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/CommentJson.java
index 0af5656..d1ce453 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/CommentJson.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/CommentJson.java
@@ -126,8 +126,11 @@
     }
     r.id = Url.encode(c.getKey().get());
     r.path = c.getKey().getParentKey().getFileName();
-    if (c.getSide() == 0) {
+    if (c.getSide() <= 0) {
       r.side = Side.PARENT;
+      if (c.getSide() < 0) {
+        r.parent = -c.getSide();
+      }
     }
     if (c.getLine() > 0) {
       r.line = c.getLine();
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/CreateDraftComment.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/CreateDraftComment.java
index bfbc828..ef95472 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/CreateDraftComment.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/CreateDraftComment.java
@@ -15,11 +15,11 @@
 package com.google.gerrit.server.change;
 
 import static com.google.gerrit.server.PatchLineCommentsUtil.setCommentRevId;
+import static com.google.gerrit.server.change.PutDraftComment.side;
 
 import com.google.common.base.Strings;
 import com.google.gerrit.common.TimeUtil;
 import com.google.gerrit.extensions.api.changes.DraftInput;
-import com.google.gerrit.extensions.client.Side;
 import com.google.gerrit.extensions.common.CommentInfo;
 import com.google.gerrit.extensions.restapi.BadRequestException;
 import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
@@ -119,7 +119,7 @@
               ChangeUtil.messageUUID(ctx.getDb())),
           line, ctx.getUser().getAccountId(), Url.decode(in.inReplyTo),
           ctx.getWhen());
-      comment.setSide(in.side == Side.PARENT ? (short) 0 : (short) 1);
+      comment.setSide(side(in));
       comment.setMessage(in.message.trim());
       comment.setRange(in.range);
       comment.setTag(in.tag);
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/FileInfoJson.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/FileInfoJson.java
index 45794fb..e0591f4 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/FileInfoJson.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/FileInfoJson.java
@@ -14,6 +14,8 @@
 
 package com.google.gerrit.server.change;
 
+import static com.google.gerrit.server.util.GitUtil.getParent;
+
 import com.google.gerrit.common.Nullable;
 import com.google.gerrit.extensions.client.DiffPreferencesInfo.Whitespace;
 import com.google.gerrit.extensions.common.FileInfo;
@@ -21,6 +23,7 @@
 import com.google.gerrit.reviewdb.client.Patch;
 import com.google.gerrit.reviewdb.client.PatchSet;
 import com.google.gerrit.reviewdb.client.RevId;
+import com.google.gerrit.server.git.GitRepositoryManager;
 import com.google.gerrit.server.patch.PatchList;
 import com.google.gerrit.server.patch.PatchListCache;
 import com.google.gerrit.server.patch.PatchListEntry;
@@ -29,17 +32,24 @@
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 
+import org.eclipse.jgit.errors.RepositoryNotFoundException;
 import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.Repository;
 
+import java.io.IOException;
 import java.util.Map;
 import java.util.TreeMap;
 
 @Singleton
 public class FileInfoJson {
   private final PatchListCache patchListCache;
+  private final GitRepositoryManager repoManager;
 
   @Inject
-  FileInfoJson(PatchListCache patchListCache) {
+  FileInfoJson(
+      PatchListCache patchListCache,
+      GitRepositoryManager repoManager) {
+    this.repoManager = repoManager;
     this.patchListCache = patchListCache;
   }
 
@@ -54,6 +64,22 @@
         ? null
         : ObjectId.fromString(base.getRevision().get());
     ObjectId b = ObjectId.fromString(revision.get());
+    return toFileInfoMap(change, a, b);
+  }
+
+  Map<String, FileInfo> toFileInfoMap(Change change, RevId revision, int parent)
+      throws RepositoryNotFoundException, IOException,
+          PatchListNotAvailableException {
+    ObjectId b = ObjectId.fromString(revision.get());
+    ObjectId a;
+    try (Repository git = repoManager.openRepository(change.getProject())) {
+      a = getParent(git, b, parent);
+    }
+    return toFileInfoMap(change, a, b);
+  }
+
+  private Map<String, FileInfo> toFileInfoMap(Change change,
+      ObjectId a, ObjectId b) throws PatchListNotAvailableException {
     PatchList list = patchListCache.get(
         new PatchListKey(a, b, Whitespace.IGNORE_NONE), change.getProject());
 
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/Files.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/Files.java
index 7f74967..35dbec1 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/Files.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/Files.java
@@ -95,6 +95,9 @@
     @Option(name = "--base", metaVar = "revision-id")
     String base;
 
+    @Option(name = "--parent", metaVar = "parent-number")
+    int parentNum;
+
     @Option(name = "--reviewed")
     boolean reviewed;
 
@@ -145,24 +148,33 @@
         return Response.ok(query(resource));
       }
 
-      PatchSet basePatchSet = null;
-      if (base != null) {
-        RevisionResource baseResource = revisions.parse(
-            resource.getChangeResource(), IdString.fromDecoded(base));
-        basePatchSet = baseResource.getPatchSet();
-      }
+      Response<Map<String, FileInfo>> r;
       try {
-        Response<Map<String, FileInfo>> r = Response.ok(fileInfoJson.toFileInfoMap(
-            resource.getChange(),
-            resource.getPatchSet().getRevision(),
-            basePatchSet));
-        if (resource.isCacheable()) {
-          r.caching(CacheControl.PRIVATE(7, TimeUnit.DAYS));
+        if (base != null) {
+          RevisionResource baseResource = revisions.parse(
+              resource.getChangeResource(), IdString.fromDecoded(base));
+          r = Response.ok(fileInfoJson.toFileInfoMap(
+              resource.getChange(),
+              resource.getPatchSet().getRevision(),
+              baseResource.getPatchSet()));
+        } else if (parentNum > 0) {
+          r = Response.ok(fileInfoJson.toFileInfoMap(
+              resource.getChange(),
+              resource.getPatchSet().getRevision(),
+              parentNum - 1));
+        } else {
+          r = Response.ok(fileInfoJson.toFileInfoMap(
+              resource.getChange(),
+              resource.getPatchSet()));
         }
-        return r;
       } catch (PatchListNotAvailableException e) {
         throw new ResourceNotFoundException(e.getMessage());
       }
+
+      if (resource.isCacheable()) {
+        r.caching(CacheControl.PRIVATE(7, TimeUnit.DAYS));
+      }
+      return r;
     }
 
     private void checkOptions() throws BadRequestException {
@@ -170,6 +182,9 @@
       if (base != null) {
         supplied++;
       }
+      if (parentNum > 0) {
+        supplied++;
+      }
       if (reviewed) {
         supplied++;
       }
@@ -177,7 +192,8 @@
         supplied++;
       }
       if (supplied > 1) {
-        throw new BadRequestException("cannot combine base, reviewed, query");
+        throw new BadRequestException(
+            "cannot combine base, parent, reviewed, query");
       }
     }
 
@@ -306,5 +322,10 @@
       this.base = base;
       return this;
     }
+
+    public ListFiles setParent(int parentNum) {
+      this.parentNum = parentNum;
+      return this;
+    }
   }
 }
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/GetDiff.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/GetDiff.java
index 3d02b83..1728c35 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/GetDiff.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/GetDiff.java
@@ -90,6 +90,9 @@
   @Option(name = "--base", metaVar = "REVISION")
   String base;
 
+  @Option(name = "--parent", metaVar = "parent-number")
+  int parentNum;
+
   @Deprecated
   @Option(name = "--ignore-whitespace")
   IgnoreWhitespace ignoreWhitespace;
@@ -121,12 +124,6 @@
   public Response<DiffInfo> apply(FileResource resource)
       throws ResourceConflictException, ResourceNotFoundException,
       OrmException, AuthException, InvalidChangeOperationException, IOException {
-    PatchSet basePatchSet = null;
-    if (base != null) {
-      RevisionResource baseResource = revisions.parse(
-          resource.getRevision().getChangeResource(), IdString.fromDecoded(base));
-      basePatchSet = baseResource.getPatchSet();
-    }
     DiffPreferencesInfo prefs = new DiffPreferencesInfo();
     if (whitespace != null) {
       prefs.ignoreWhitespace = whitespace;
@@ -138,13 +135,35 @@
     prefs.context = context;
     prefs.intralineDifference = intraline;
 
-    try {
-      PatchScriptFactory psf = patchScriptFactoryFactory.create(
+    PatchScriptFactory psf;
+    PatchSet basePatchSet = null;
+    if (base != null) {
+      RevisionResource baseResource = revisions.parse(
+          resource.getRevision().getChangeResource(), IdString.fromDecoded(base));
+      basePatchSet = baseResource.getPatchSet();
+      psf = patchScriptFactoryFactory.create(
           resource.getRevision().getControl(),
           resource.getPatchKey().getFileName(),
-          basePatchSet != null ? basePatchSet.getId() : null,
+          basePatchSet.getId(),
           resource.getPatchKey().getParentKey(),
           prefs);
+    } else if (parentNum > 0) {
+      psf = patchScriptFactoryFactory.create(
+          resource.getRevision().getControl(),
+          resource.getPatchKey().getFileName(),
+          parentNum - 1,
+          resource.getPatchKey().getParentKey(),
+          prefs);
+    } else {
+      psf = patchScriptFactoryFactory.create(
+          resource.getRevision().getControl(),
+          resource.getPatchKey().getFileName(),
+          null,
+          resource.getPatchKey().getParentKey(),
+          prefs);
+    }
+
+    try {
       psf.setLoadHistory(false);
       psf.setLoadComments(context != DiffPreferencesInfo.WHOLE_FILE_CONTEXT);
       PatchScript ps = psf.call();
@@ -272,6 +291,11 @@
     return this;
   }
 
+  public GetDiff setParent(int parentNum) {
+    this.parentNum = parentNum;
+    return this;
+  }
+
   public GetDiff setContext(int context) {
     this.context = context;
     return this;
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java
index c4cdab5..9370e21 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java
@@ -17,6 +17,7 @@
 import static com.google.common.base.MoreObjects.firstNonNull;
 import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.gerrit.server.PatchLineCommentsUtil.setCommentRevId;
+import static com.google.gerrit.server.change.PutDraftComment.side;
 import static com.google.gerrit.server.notedb.ReviewerStateInternal.REVIEWER;
 import static java.nio.charset.StandardCharsets.UTF_8;
 
@@ -426,7 +427,7 @@
           }
           e.setStatus(PatchLineComment.Status.PUBLISHED);
           e.setWrittenOn(ctx.getWhen());
-          e.setSide(c.side == Side.PARENT ? (short) 0 : (short) 1);
+          e.setSide(side(c));
           setCommentRevId(e, patchListCache, ctx.getChange(), ps);
           e.setMessage(c.message);
           e.setTag(in.tag);
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/PutDraftComment.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/PutDraftComment.java
index 1d756f4..0329fa7 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/PutDraftComment.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/PutDraftComment.java
@@ -19,6 +19,7 @@
 import com.google.common.base.Optional;
 import com.google.gerrit.common.TimeUtil;
 import com.google.gerrit.extensions.api.changes.DraftInput;
+import com.google.gerrit.extensions.client.Comment;
 import com.google.gerrit.extensions.client.Side;
 import com.google.gerrit.extensions.common.CommentInfo;
 import com.google.gerrit.extensions.restapi.BadRequestException;
@@ -163,7 +164,7 @@
   private static PatchLineComment update(PatchLineComment e, DraftInput in,
       Timestamp when) {
     if (in.side != null) {
-      e.setSide(in.side == Side.PARENT ? (short) 0 : (short) 1);
+      e.setSide(side(in));
     }
     if (in.inReplyTo != null) {
       e.setParentUuid(Url.decode(in.inReplyTo));
@@ -180,4 +181,11 @@
     }
     return e;
   }
+
+  static short side(Comment c) {
+    if (c.side == Side.PARENT) {
+      return (short) (c.parent == null ? 0 : -c.parent.shortValue());
+    }
+    return 1;
+  }
 }
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/notedb/ChangeNoteUtil.java b/gerrit-server/src/main/java/com/google/gerrit/server/notedb/ChangeNoteUtil.java
index f9bad6f..4c1a734 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/notedb/ChangeNoteUtil.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/notedb/ChangeNoteUtil.java
@@ -82,6 +82,7 @@
   private static final String FILE = "File";
   private static final String LENGTH = "Bytes";
   private static final String PARENT = "Parent";
+  private static final String PARENT_NUMBER = "Parent-number";
   private static final String PATCH_SET = "Patch-set";
   private static final String REVISION = "Revision";
   private static final String UUID = "UUID";
@@ -151,11 +152,13 @@
     int sizeOfNote = note.length;
     byte[] psb = PATCH_SET.getBytes(UTF_8);
     byte[] bpsb = BASE_PATCH_SET.getBytes(UTF_8);
+    byte[] bpn = PARENT_NUMBER.getBytes(UTF_8);
 
     RevId revId = new RevId(parseStringField(note, p, changeId, REVISION));
     String fileName = null;
     PatchSet.Id psId = null;
     boolean isForBase = false;
+    Integer parentNumber = null;
 
     while (p.value < sizeOfNote) {
       boolean matchPs = match(note, p, psb);
@@ -168,13 +171,16 @@
         fileName = null;
         psId = parsePsId(note, p, changeId, BASE_PATCH_SET);
         isForBase = true;
+        if (match(note, p, bpn)) {
+          parentNumber = parseParentNumber(note, p, changeId);
+        }
       } else if (psId == null) {
         throw parseException(changeId, "missing %s or %s header",
             PATCH_SET, BASE_PATCH_SET);
       }
 
-      PatchLineComment c =
-          parseComment(note, p, fileName, psId, revId, isForBase, status);
+      PatchLineComment c = parseComment(
+          note, p, fileName, psId, revId, isForBase, parentNumber, status);
       fileName = c.getKey().getParentKey().getFileName();
       if (!seen.add(c.getKey())) {
         throw parseException(
@@ -187,7 +193,7 @@
 
   private PatchLineComment parseComment(byte[] note, MutableInteger curr,
       String currentFileName, PatchSet.Id psId, RevId revId, boolean isForBase,
-      Status status) throws ConfigInvalidException {
+      Integer parentNumber, Status status) throws ConfigInvalidException {
     Change.Id changeId = psId.getParentKey();
 
     // Check if there is a new file.
@@ -235,7 +241,13 @@
         range.getEndLine(), aId, parentUUID, commentTime);
     plc.setMessage(message);
     plc.setTag(tag);
-    plc.setSide((short) (isForBase ? 0 : 1));
+
+    if (isForBase) {
+      plc.setSide((short) (parentNumber == null ? 0 : -parentNumber));
+    } else {
+      plc.setSide((short) 1);
+    }
+
     if (range.getStartCharacter() != -1) {
       plc.setRange(range);
     }
@@ -333,6 +345,23 @@
     return new PatchSet.Id(changeId, patchSetId);
   }
 
+  private static Integer parseParentNumber(byte[] note, MutableInteger curr,
+      Change.Id changeId) throws ConfigInvalidException {
+    checkHeaderLineFormat(note, curr, PARENT_NUMBER, changeId);
+
+    int start = RawParseUtils.endOfFooterLineKey(note, curr.value) + 1;
+    MutableInteger i = new MutableInteger();
+    int parentNumber = RawParseUtils.parseBase10(note, start, i);
+    int endOfLine = RawParseUtils.nextLF(note, curr.value);
+    if (i.value != endOfLine - 1) {
+      throw parseException(changeId, "could not parse %s", PARENT_NUMBER);
+    }
+    checkResult(parentNumber, "parent number", changeId);
+    curr.value = endOfLine;
+    return Integer.valueOf(parentNumber);
+
+  }
+
   private static String parseFilename(byte[] note, MutableInteger curr,
       Change.Id changeId) throws ConfigInvalidException {
     checkHeaderLineFormat(note, curr, FILE, changeId);
@@ -461,10 +490,13 @@
         PatchLineComment first = psComments.get(0);
 
         short side = first.getSide();
-        appendHeaderField(writer, side == 0
+        appendHeaderField(writer, side <= 0
             ? BASE_PATCH_SET
             : PATCH_SET,
             Integer.toString(psId.get()));
+        if (side < 0) {
+          appendHeaderField(writer, PARENT_NUMBER, Integer.toString(-side));
+        }
 
         String currentFilename = null;
 
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListCache.java b/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListCache.java
index 2099376..8a2403f 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListCache.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListCache.java
@@ -28,7 +28,7 @@
   PatchList get(Change change, PatchSet patchSet)
       throws PatchListNotAvailableException;
 
-  ObjectId getOldId(Change change, PatchSet patchSet)
+  ObjectId getOldId(Change change, PatchSet patchSet, Integer parentNum)
       throws PatchListNotAvailableException;
 
   IntraLineDiff getIntraLineDiff(IntraLineDiffKey key,
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListCacheImpl.java b/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListCacheImpl.java
index 7c8f19f..abafad7 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListCacheImpl.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListCacheImpl.java
@@ -103,6 +103,17 @@
   @Override
   public PatchList get(Change change, PatchSet patchSet)
       throws PatchListNotAvailableException {
+    return get(change, patchSet, null);
+  }
+
+  @Override
+  public ObjectId getOldId(Change change, PatchSet patchSet, Integer parentNum)
+      throws PatchListNotAvailableException {
+    return get(change, patchSet, parentNum).getOldId();
+  }
+
+  private PatchList get(Change change, PatchSet patchSet, Integer parentNum)
+      throws PatchListNotAvailableException {
     Project.NameKey project = change.getProject();
     if (patchSet.getRevision() == null) {
       throw new PatchListNotAvailableException(
@@ -110,13 +121,10 @@
     }
     ObjectId b = ObjectId.fromString(patchSet.getRevision().get());
     Whitespace ws = Whitespace.IGNORE_NONE;
-    return get(new PatchListKey(null, b, ws), project);
-  }
-
-  @Override
-  public ObjectId getOldId(Change change, PatchSet patchSet)
-      throws PatchListNotAvailableException {
-    return get(change, patchSet).getOldId();
+    if (parentNum != null) {
+      return get(PatchListKey.againstParentNum(parentNum, b, ws), project);
+    }
+    return get(PatchListKey.againstDefaultBase(b, ws), project);
   }
 
   @Override
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListKey.java b/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListKey.java
index b04558d..1fb0503 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListKey.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListKey.java
@@ -32,9 +32,10 @@
 import java.io.ObjectInputStream;
 import java.io.ObjectOutputStream;
 import java.io.Serializable;
+import java.util.Objects;
 
 public class PatchListKey implements Serializable {
-  static final long serialVersionUID = 20L;
+  static final long serialVersionUID = 21L;
 
   public static final BiMap<Whitespace, Character> WHITESPACE_TYPES = ImmutableBiMap.of(
       Whitespace.IGNORE_NONE, 'N',
@@ -46,7 +47,36 @@
     checkState(WHITESPACE_TYPES.size() == Whitespace.values().length);
   }
 
+  public static PatchListKey againstDefaultBase(AnyObjectId newId,
+      Whitespace ws) {
+    return new PatchListKey(null, newId, ws);
+  }
+
+  public static PatchListKey againstParentNum(int parentNum, AnyObjectId newId,
+      Whitespace ws) {
+    return new PatchListKey(parentNum, newId, ws);
+  }
+
+  /**
+   * Old patch-set ID
+   * <p>
+   * When null, it represents the Base of the newId for a non-merge commit.
+   * <p>
+   * When newId is a merge commit, null value of the oldId represents either
+   * the auto-merge commit of the newId or a parent commit of the newId.
+   * These two cases are distinguished by the parentNum.
+   */
   private transient ObjectId oldId;
+
+  /**
+   * 1-based parent number when newId is a merge commit
+   * <p>
+   * For the auto-merge case this field is null.
+   * <p>
+   * Used only when oldId is null and newId is a merge commit
+   */
+  private transient Integer parentNum;
+
   private transient ObjectId newId;
   private transient Whitespace whitespace;
 
@@ -56,12 +86,24 @@
     whitespace = ws;
   }
 
+  private PatchListKey(int parentNum, AnyObjectId b, Whitespace ws) {
+    this.parentNum = Integer.valueOf(parentNum);
+    newId = b.copy();
+    whitespace = ws;
+  }
+
   /** Old side commit, or null to assume ancestor or combined merge. */
   @Nullable
   public ObjectId getOldId() {
     return oldId;
   }
 
+  /** Parent number (old side) of the new side (merge) commit */
+  @Nullable
+  public Integer getParentNum() {
+    return parentNum;
+  }
+
   /** New side commit name. */
   public ObjectId getNewId() {
     return newId;
@@ -73,24 +115,16 @@
 
   @Override
   public int hashCode() {
-    int h = 0;
-
-    if (oldId != null) {
-      h = h * 31 + oldId.hashCode();
-    }
-
-    h = h * 31 + newId.hashCode();
-    h = h * 31 + whitespace.name().hashCode();
-
-    return h;
+    return Objects.hash(oldId, parentNum, newId, whitespace);
   }
 
   @Override
   public boolean equals(final Object o) {
     if (o instanceof PatchListKey) {
-      final PatchListKey k = (PatchListKey) o;
-      return eq(oldId, k.oldId) //
-          && eq(newId, k.newId) //
+      PatchListKey k = (PatchListKey) o;
+      return Objects.equals(oldId, k.oldId)
+          && Objects.equals(parentNum, k.parentNum)
+          && Objects.equals(newId, k.newId)
           && whitespace == k.whitespace;
     }
     return false;
@@ -109,15 +143,9 @@
     return n.toString();
   }
 
-  private static boolean eq(final ObjectId a, final ObjectId b) {
-    if (a == null && b == null) {
-      return true;
-    }
-    return a != null && b != null && AnyObjectId.equals(a, b);
-  }
-
   private void writeObject(final ObjectOutputStream out) throws IOException {
     writeCanBeNull(out, oldId);
+    out.writeInt(parentNum == null ? 0 : parentNum);
     writeNotNull(out, newId);
     Character c = WHITESPACE_TYPES.get(whitespace);
     if (c == null) {
@@ -128,6 +156,8 @@
 
   private void readObject(final ObjectInputStream in) throws IOException {
     oldId = readCanBeNull(in);
+    int n = in.readInt();
+    parentNum = n == 0 ? null : Integer.valueOf(n);
     newId = readNotNull(in);
     char t = in.readChar();
     whitespace = WHITESPACE_TYPES.inverse().get(t);
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListLoader.java b/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListLoader.java
index 90bebfc..eb817a4 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListLoader.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListLoader.java
@@ -163,11 +163,11 @@
       List<DiffEntry> diffEntries = df.scan(aTree, bTree);
 
       Set<String> paths = null;
-      if (key.getOldId() != null) {
-        PatchListKey newKey =
-            new PatchListKey(null, key.getNewId(), key.getWhitespace());
-        PatchListKey oldKey =
-            new PatchListKey(null, key.getOldId(), key.getWhitespace());
+      if (key.getOldId() != null && b.getParentCount() == 1) {
+        PatchListKey newKey = PatchListKey.againstDefaultBase(
+            key.getNewId(), key.getWhitespace());
+        PatchListKey oldKey = PatchListKey.againstDefaultBase(
+            key.getOldId(), key.getWhitespace());
         paths = FluentIterable
             .from(patchListCache.get(newKey, project).getPatches())
             .append(patchListCache.get(oldKey, project).getPatches())
@@ -331,6 +331,11 @@
         return r;
       }
       case 2:
+        if (key.getParentNum() != null) {
+          RevCommit r = b.getParent(key.getParentNum() - 1);
+          rw.parseBody(r);
+          return r;
+        }
         return autoMerger.merge(repo, rw, ins, b, mergeStrategy);
       default:
         // TODO(sop) handle an octopus merge.
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchScriptFactory.java b/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchScriptFactory.java
index b7ca69d4..1da353b 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchScriptFactory.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchScriptFactory.java
@@ -15,6 +15,7 @@
 package com.google.gerrit.server.patch;
 
 import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.gerrit.server.util.GitUtil.getParent;
 
 import com.google.common.base.Optional;
 import com.google.gerrit.common.Nullable;
@@ -44,9 +45,9 @@
 import com.google.gerrit.server.project.InvalidChangeOperationException;
 import com.google.gerrit.server.project.NoSuchChangeException;
 import com.google.gwtorm.server.OrmException;
-import com.google.inject.Inject;
 import com.google.inject.Provider;
 import com.google.inject.assistedinject.Assisted;
+import com.google.inject.assistedinject.AssistedInject;
 
 import org.eclipse.jgit.errors.RepositoryNotFoundException;
 import org.eclipse.jgit.lib.ObjectId;
@@ -70,6 +71,13 @@
         @Assisted("patchSetA") PatchSet.Id patchSetA,
         @Assisted("patchSetB") PatchSet.Id patchSetB,
         DiffPreferencesInfo diffPrefs);
+
+    PatchScriptFactory create(
+        ChangeControl control,
+        String fileName,
+        int parentNum,
+        PatchSet.Id patchSetB,
+        DiffPreferencesInfo diffPrefs);
   }
 
   private static final Logger log =
@@ -86,6 +94,8 @@
   private final String fileName;
   @Nullable
   private final PatchSet.Id psa;
+  @Nullable
+  private final int parentNum;
   private final PatchSet.Id psb;
   private final DiffPreferencesInfo diffPrefs;
   private final ChangeEditUtil editReader;
@@ -103,7 +113,7 @@
   private List<Patch> history;
   private CommentDetail comments;
 
-  @Inject
+  @AssistedInject
   PatchScriptFactory(GitRepositoryManager grm,
       PatchSetUtil psUtil,
       Provider<PatchScriptBuilder> builderFactory,
@@ -129,14 +139,45 @@
 
     this.fileName = fileName;
     this.psa = patchSetA;
+    this.parentNum = -1;
     this.psb = patchSetB;
     this.diffPrefs = diffPrefs;
 
     changeId = patchSetB.getParentKey();
-    checkArgument(
-        patchSetA == null || patchSetA.getParentKey().equals(changeId),
-        "cannot compare PatchSets from different changes: %s and %s",
-        patchSetA, patchSetB);
+  }
+
+  @AssistedInject
+  PatchScriptFactory(GitRepositoryManager grm,
+      PatchSetUtil psUtil,
+      Provider<PatchScriptBuilder> builderFactory,
+      PatchListCache patchListCache,
+      ReviewDb db,
+      AccountInfoCacheFactory.Factory aicFactory,
+      PatchLineCommentsUtil plcUtil,
+      ChangeEditUtil editReader,
+      @Assisted ChangeControl control,
+      @Assisted String fileName,
+      @Assisted int parentNum,
+      @Assisted PatchSet.Id patchSetB,
+      @Assisted DiffPreferencesInfo diffPrefs) {
+    this.repoManager = grm;
+    this.psUtil = psUtil;
+    this.builderFactory = builderFactory;
+    this.patchListCache = patchListCache;
+    this.db = db;
+    this.control = control;
+    this.aicFactory = aicFactory;
+    this.plcUtil = plcUtil;
+    this.editReader = editReader;
+
+    this.fileName = fileName;
+    this.psa = null;
+    this.parentNum = parentNum;
+    this.psb = patchSetB;
+    this.diffPrefs = diffPrefs;
+
+    changeId = patchSetB.getParentKey();
+    checkArgument(parentNum >= 0, "parentNum must be >= 0");
   }
 
   public void setLoadHistory(boolean load) {
@@ -151,7 +192,9 @@
   public PatchScript call() throws OrmException, NoSuchChangeException,
       LargeObjectException, AuthException,
       InvalidChangeOperationException, IOException {
-    validatePatchSetId(psa);
+    if (parentNum < 0) {
+      validatePatchSetId(psa);
+    }
     validatePatchSetId(psb);
 
     change = control.getChange();
@@ -163,15 +206,19 @@
         ? new PatchSet(psb)
         : psUtil.get(db, control.getNotes(), psb);
 
-    aId = psEntityA != null ? toObjectId(psEntityA) : null;
-    bId = toObjectId(psEntityB);
-
     if ((psEntityA != null && !control.isPatchVisible(psEntityA, db)) ||
         (psEntityB != null && !control.isPatchVisible(psEntityB, db))) {
       throw new NoSuchChangeException(changeId);
     }
 
     try (Repository git = repoManager.openRepository(project)) {
+      bId = toObjectId(psEntityB);
+      if (parentNum < 0) {
+        aId = psEntityA != null ? toObjectId(psEntityA) : null;
+      } else {
+        aId = getParent(git, bId, parentNum);
+      }
+
       try {
         final PatchList list = listFor(keyFor(diffPrefs.ignoreWhitespace));
         final PatchScriptBuilder b = newBuilder(list, git);
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/util/GitUtil.java b/gerrit-server/src/main/java/com/google/gerrit/server/util/GitUtil.java
new file mode 100644
index 0000000..2d1e1fa
--- /dev/null
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/util/GitUtil.java
@@ -0,0 +1,51 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.util;
+
+import org.eclipse.jgit.errors.IncorrectObjectTypeException;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevWalk;
+
+import java.io.IOException;
+
+public class GitUtil {
+
+  /**
+   * @param git
+   * @param commitId
+   * @param parentNum
+   * @return the {@code paretNo} parent of given commit or {@code null}
+   *             when {@code parentNo} exceed number of {@code commitId} parents.
+   * @throws IncorrectObjectTypeException
+   *             the supplied id is not a commit or an annotated tag.
+   * @throws IOException
+   *             a pack file or loose object could not be read.
+   */
+  public static RevCommit getParent(Repository git,
+      ObjectId commitId, int parentNum) throws IOException {
+    try (RevWalk walk = new RevWalk(git)) {
+      RevCommit commit = walk.parseCommit(commitId);
+      if (commit.getParentCount() > parentNum) {
+        return commit.getParent(parentNum);
+      }
+    }
+    return null;
+  }
+
+  private GitUtil() {
+  }
+}