diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index a5bfd1023d050e94adafbb0db149a470848b8d01..c0467f034ff29c8b98476f6f10349483fdb1ff63 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2911,7 +2911,18 @@ function confirm_form($form, $question, $path, $description = NULL, $yes = NULL,
 }
 
 /**
- * Determines if the current user is in compact mode.
+ * Determines whether the current user is in compact mode.
+ *
+ * Compact mode shows certain administration pages with less description text,
+ * such as the configuration page and the permissions page.
+ *
+ * Whether the user is in compact mode is determined by a cookie, which is set
+ * for the user by system_admin_compact_page().
+ *
+ * If the user does not have the cookie, the default value is given by the
+ * system variable 'admin_compact_mode', which itself defaults to FALSE. This
+ * does not have a user interface to set it: it is a hidden variable which can
+ * be set in the settings.php file.
  *
  * @return
  *   TRUE when in compact mode, FALSE when in expanded mode.