Required for glass installations, this product can be used with the SureFlap Microchip Pet Door. When mounting in glass, this accessory allows mounting in circular holes with a diameter of 11 9/16" (min) to 12 5/8" (max). The total diameter of the mounting adaptor is 15". The Pet Door Mounting Adaptor includes two parts, one for the exterior and one for the interior of your home. The Pet Door Mounting Adaptor is not required for wall installations, as the pet door has internal screw holes to enable fixing to a wall.