Stray pets can often be a big problem, but this highly proven design provides a reliable solution. Your cat wears a small magnet that doubles as an identification tag. A sensitive, fast-reacting circuit in the cat flap guarantees instant entry for selected cats of all size and temperament. An easy to use 4 Way locking system enables complete control over your cats movements. Any animal can exit the door, but animal will require a magnet to re-enter. Self lining to 2 3/8 (60mm) - ideal for all doors. One 9 volt battery not included."