diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -1114,8 +1114,8 @@ def diff(ui, repo, *pats, **opts):
     revisions are specified, the working directory files are compared
     to its parent.
 
-    Alternatively you can specify -c/--change with a revision to use
-    as a baseline.
+    Alternatively you can specify -c/--change with a revision to see
+    the changes in that changeset relative to its first parent.
 
     Without the -a/--text option, diff will avoid generating diffs of
     files it detects as binary. With -a, diff will generate a diff
diff --git a/tests/test-help.out b/tests/test-help.out
--- a/tests/test-help.out
+++ b/tests/test-help.out
@@ -383,8 +383,8 @@ diff repository (or selected files)
     compared to the working directory, and, when no revisions are specified,
     the working directory files are compared to its parent.
 
-    Alternatively you can specify -c/--change with a revision to use as a
-    baseline.
+    Alternatively you can specify -c/--change with a revision to see the
+    changes in that changeset relative to its first parent.
 
     Without the -a/--text option, diff will avoid generating diffs of files it
     detects as binary. With -a, diff will generate a diff anyway, probably