PetSafe 4-0111-11 Medium Single Flap Replacement is also called as single flap replacement flap which is all one piece and can only be used for Classic PetSafe Pet Door. Each pet door needs a flap in order to protect your cat or dogs from the bad weather and their enemies. Features: Single Flap Replacement. Use with PetSafe Classic Door. 8 1/2 inch x 12 inch Opening.