diff --git a/lisp/startup.el b/lisp/startup.el
index 19ef664a2fe..f90ffeba0ef 100644
--- a/lisp/startup.el
+++ b/lisp/startup.el
@@ -815,6 +815,10 @@ or `CVS', and any subdirectory that contains a file named `.nosearch'."
     ;; Run the site-start library if it exists.  The point of this file is
     ;; that it is run before .emacs.  There is no point in doing this after
     ;; .emacs; that is useless.
+    ;; Note that user-init-file is nil at this point.  Code that might
+    ;; be loaded from site-run-file and wants to test if -q was given
+    ;; should check init-file-user instead, since that is already set.
+    ;; See cus-edit.el for an example.
     (if site-run-file
 	(load site-run-file t t))