diff --git a/database/updates.inc b/database/updates.inc
index 022ddf07792c857cedb090245d5568d0e0e15c9b..e8103ac898023941c22f414f47b86d63f7f19ad2 100644
--- a/database/updates.inc
+++ b/database/updates.inc
@@ -1,6 +1,9 @@
 <?php
 // $Id$
 
+// SCHEMA and SCHEMA_MIN represent the minimum and maximum N modulename_update_N.
+// Update SCHEMA when a new update function is added.  Update SCHEMA_MIN when old
+// update functions are removed.
 function system_version($type) {
   switch ($type) {
     case SCHEMA: