"Your pet can come and go as he pleases with the PetSafe Electronic SmartDoor. Using radio-frequency technology, the SmartDoor reads the unique signal of a SmartKey (included) worn on your pet's collar and triggers the battery power-driven flap to unlock. When the SmartDoor no longer senses your pet's SmartKey, the flap automatically locks back into place, reducing the risk of unwanted animals entering your home. SmartDoor offers three programmable modes: locked, unlocked and automatic (access only for pets wearing a SmartKey). Programmable for up to five pets. Uses four D batteries (not included). Imported. Available: Small (15-5/8""H x 9""W frame, 7-7/8""H x 5-1/2""W flap) Large (23-5/8""H x 16-1/8""W frame, 16""H x11""W flap)"