Measurements for the back plates are for the sign blade going onto the back plate.  Back plates are oversized to provide a 1" wide border on all sides.  For example, BP-24OCT is actually 26" wide, allowing for a 1" border around a 24" octagonal sign. The traffic back plates come undrilled for custom mounting. 

To order a custom back plate, use the item number followed by the height and width you need for your custom sign application.  For example to order back plate for a 15"x30" sign, the item number would be BP-1530.