A cat door or cat flap is a small portal in a wall, window or human door to allow pets to enter and exit a house on their own without needing a person to open the door. Originally simple holes, the modern form is a hinged and often spring-loaded panel or flexible flap, and some are electronically controlled. They offer a degree of protection against wind, rain, and larger-bodied intruders entering the house. A related concept is the pet gate, which is easy for humans to open but acts as a secure pet barrier.
The simplest type are bottom-weighted flaps hung from the top of the opening, which swing shut on their own, but may be spring-aided to increase wind resistance. These flaps often feature magnets around the edges to help keep the door closed against foul weather. Some doors have side-mounted hinges and more saloon-type of door. These pet doors usually have a spring or other contrivance to force their closure after the pet has gone through. Instead of a rubber flap, saloon style doors are often made from plastic, acrylic, or plexiglas, and the panels are fitted with weatherseal to help keep weather outside.
Another common feature is an adjustable catch to restrict the opening of the device to either one direction or the other; for example, to allow the pet to come in for the night, but not go out again until the owner releases the catch the next morning. Some pets, mostly cats with their retractile claws and flexible paws, learn to circumvent one-way pet doors, especially the "flap-within-flap" design.
Most also have a locking mechanism of some kind, and can be closed off by sliding a rigid plate into parallel rails on the left and right of the interior side of the pet door to close it off, e.g. during bad weather or when the owners are travelling with their pets.
Pet doors are generally designed to be safe for any pet. The panels are often designed with soft vinyl that does not trap or injure the animal. Cheap, easily replaceable pet doors are made from plastic and as such may not always be robust enough for large, boisterous pets.
Pet doors are most often fitted in a plywood or plastic panelled door, into which it is straightforward to cut a large round hole, but can also be fitted in brickwork or (if a sealed unit is obtained with the hole already provided) in a double glazed door. The latter is a relatively expensive option but may be the only alternative in some cases.
In rural areas, cat doors (often simple holes) in the walls, doors or even roofs of grain and flour storage spaces have long been used to welcome feral cats to hunt rodent pests that feed on these stores.
The 14th-century English writer Geoffrey Chaucer described a simple cat hole in the "Miller's Tale" from his Canterbury Tales (late 14th century). In the narrative, a servant whose knocks go unanswered uses the cat door to peek in:
An hole he foond, ful lowe upon a bord
Ther as the cat was wont in for to crepe,
And at the hole he looked in ful depe,
And at the last he hadde of hym a sighte.
In an apparent early modern example of urban legend, the invention of the pet door was attributed to Isaac Newton (1642–1727) in a story (authored anonymously and published in a column of anecdotes in 1893) to the effect that Newton foolishly made a large hole for his adult cat and a small one for her kittens, not realizing the kittens would follow the mother through the large one. Two Newton biographers cite passages saying that Newton kept "neither cat nor dog in his chamber". Yet over 60 years earlier, a member of Newton's social circles at Trinity, one J. M. F. Wright, reported this same story (from an unknown source) in his 1827 memoir, adding: "Whether this account be true or false, indisputably true is it that there are in the door to this day two plugged holes of the proper dimensions for the respective egresses of cat and kitten."
Modern cat flaps are popular in some countries even in urban environments, particularly the United Kingdom where it is estimated that about 74% of cats have access to the outdoors.
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