diff --git a/lisp/simple.el b/lisp/simple.el
index 223e0116ae..5977f6c035 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -1713,7 +1713,11 @@ repeat-complex-command
 	(error "There are no previous complex commands to repeat")))))
 
 
-(defvar extended-command-history nil)
+(defvar extended-command-history nil
+  "History list for extended command name arguments.
+It is used by `execute-extended-command', and its maximum length
+is determined by the value of `history-length', which see.")
+
 (defvar execute-extended-command--last-typed nil)
 
 (defun read-extended-command ()