Index: core.php
===================================================================
RCS file: /cvs/drupal-contrib/contributions/docs/developer/hooks/core.php,v
retrieving revision 1.211
diff -u -u -r1.211 core.php
--- core.php	13 Oct 2008 23:18:37 -0000	1.211
+++ core.php	22 Oct 2008 13:11:49 -0000
@@ -652,6 +652,31 @@
 }
 
 /**
+ * Perform alterations to existing database schemas.
+ *
+ * When a module modifies the database structure of another module (by
+ * changing, adding or removing fields, keys or indexes), it should
+ * implement hook_schema_alter() to update the default $schema to take
+ * it's changes into account.
+ *
+ * See hook_schema() for details on the schema definition structure.
+ *
+ * @param $schema
+ *   Nested array describing the schemas for all modules.
+ * @return
+ *   None.
+ */
+function hook_schema_alter(&$schema) {
+  // Add field to existing schema.
+  $schema['users']['fields']['timezone_id'] = array(
+    'type' => 'int',
+    'not null' => TRUE,
+    'default' => 0,
+    'description' => t('Per-user timezone configuration.'),
+  );
+}
+
+/**
  * Perform alterations before a form is rendered.
  *
  * One popular use of this hook is to add form elements to the node form. When