This chapter explains hook_page_alter(), which is another alter hook new to Drupal 7 and to the theme layer. Each page rendered by Drupal is contained within an array where the theme is able to go in and rearrange, delete or duplicate specific page elements.
Check your version
This video covers a topic in Drupal 7 which may or may not be the version you're using. We're keeping this tutorial online as a courtesy to users of Drupal 7, but we consider it archived.