Cat flap

A cat flap (British English) or kitty door (American English) is a hinged flap set into a door, wall or window to allow cats to enter and exit a house on their own without needing a human to open the door, while offering a degree of protection against wind and rain entering the dwelling. Doggie doors similarly exist for dogs.

Cat flaps are popular in some countries, particularly the United Kingdom where it is believed that about 90% of cats have access to the outdoors^[1].

The Oxford English Dictionary records the first use of cat flap in 1957 and the first use of cat door in 1959. ^[2] The Webster's Dictionary of English lists cat door as a synonym of cat flap. ^[3]

The invention of the cat flap is frequently attributed to Isaac Newton on the World Wide Web.^[4] However, author Charles R. Gibson, writing about the life of Newton in 1921, recorded that "To this day students at Cambridge are told how there were two holes cut in the door of Newton's chamber: one hole, much larger than the other, for the use of his cat, the smaller one for the convenience of the kitten", but states that this goes in the face of "...a letter written by Newton's assistant [who] gives us the following information, which is direct evidence and not mere hearsay. `He kept neither dog nor cat in his chamber...'"^[5]

The simplest cat flaps are just weighted flaps, but these may be blown open or may make a rattling noise in the wind. A magnet on the flap can hold the door in place and allow access. Some flaps also have adjustable catches to restrict the opening of the flap in one direction or the other — for example to allow the cat to come in, but not go out again. More sophisticated flaps have electromagnetic catches that can detect a matching permanent magnet worn on the cat's collar that will unlock the flap as the cat approaches, thus preventing any other animals from using the flap and entering the house. Flaps with infrared locks open only when a collar-mounted device transmits the correct code to the flap, allowing cat owners to have multiple flaps that different cats can use. A cat door operates like a house door rather than a flap, in that it is either hinged or spring operated opening sideways like a door. The Plexidor is one type of pet door recognised for the quiet way it operates and because it offers superior weatherproofing than most other pet doors available.

Some cats can circumvent one-way flap-mounted-in-flap designs by pulling at the bottom of the inner flap. Cats can carry rodents into a dwelling through flaps; one cat owner has gone to technological extremes to solve this problem, employing a webcam and image recognition software ^[6].