(defcustom custom-file nil
"File used for storing customization information.
The default is nil, which means to use your init file
-as specified by `user-init-file'.
+as specified by `user-init-file'. To make this feature work,
+you'll need to put something in your init file to specify
+the value of `custom-file'. Just customizing the variable
+won't suffice, because Emacs won't know which file to load
+unless the init file sets `custom-file'.
When you change this variable, look in the previous custom file
\(usually your init file) for the forms `(custom-set-variables ...)'