diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -625,7 +625,7 @@
 
 @predicate('commonancestors(set)', safe=True)
 def commonancestors(repo, subset, x):
-    """Returns all common ancestors of the set.
+    """Changesets that are ancestors of every changeset in set.
     """
     startrevs = getset(repo, fullreposet(repo), x)
     if not startrevs: