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    'type'      => 'page_layout',                // - type, tell the system this is a page_layout, not optional!

    'admin css' => 'three_col_grail.admin.css',  // - admin css, optional, in AT Core three_col_grail.admin.css loads all the CSS for all layouts.

    'device_groups' => array(                    // - device_groups, define which device groups this layout can work with, can be one or more of:

      'tablet_landscape',                        //   What you enter here will dictate the device groups it shows for in theme settings.

     ),

  );