It is designed to fit single and double glazing. It has the added feature of the flap being able to swing higher to reduce the problem of broken flaps.  

• Generous flap size: 240mm x 215mm (9 29/64″   x  8  15/32″)
• Overall size: 340mm x 340mm (13 25/64″ x 13 25/64″ )
• Application: Suitable for single and double glazing. Glass thickness 4mm to 32mm (5/32″ – 1 17/64″)