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 the animal requires the magnet to re-enter. Self lining to 2 3/8 (60mm) - ideal for all doors. One 9 volt battery not included". Helpful tips: 1) The collar magnet should be attached directly to the cat’s collar with the paw print symbol facing forwards (see fig 4 of the User Manual). Not doing so will affect this product’s Performance. 2) Ensure that the four way lock is fully seated in the correct position, i.e. in order for pets to go in and out it should be in the unlocked position.