diff --git a/Documentation/revisions.txt b/Documentation/revisions.txt
index b0f72206a..242682af0 100644
--- a/Documentation/revisions.txt
+++ b/Documentation/revisions.txt
@@ -239,11 +239,13 @@ To summarize:
 
 '<rev1>..<rev2>'::
 	Include commits that are reachable from <rev2> but exclude
-	those that are reachable from <rev1>.
+	those that are reachable from <rev1>.  When either <rev1> or
+	<rev2> is omitted, it defaults to 'HEAD'.
 
 '<rev1>\...<rev2>'::
 	Include commits that are reachable from either <rev1> or
-	<rev2> but exclude those that are reachable from both.
+	<rev2> but exclude those that are reachable from both.  When
+	either <rev1> or <rev2> is omitted, it defaults to 'HEAD'.
 
 '<rev>{caret}@', e.g. 'HEAD{caret}@'::
   A suffix '{caret}' followed by an at sign is the same as listing