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 Oxford English Dictionary records the first use of the phrase 'cat flap' in 1957 and 'cat door' in 1959, but the idea is much older.ДЄ gatera (farm cat hole) in RincГіn de Ademuz, Valencia, Spain Innovation has contributed to a new generation of more expensive pet doors making use of specific materials, automation, time control devices, and/or sophisticated sensors to deal with common problems like poor insulation and drafts, higher noise levels, insufficient pet safety and access difficulties. Removable pet doors suitable for sliding glass doors are also available. The latter is a relatively expensive option but may be the only alternative in some cases. Pet doors are most often fitted in a plywood or plastic paneled 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. Cheap, easily replaceable pet doors are made from plastic and may not always be robust enough for large, boisterous pets. The panels are often designed with soft vinyl that does not trap or injure the animal. Pet doors are generally designed to be safe for any type of pet. 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, useful during bad weather or when the owners are traveling with their pets. Some pets, mostly cats with their retractile claws and flexible paws, learn to circumvent one-way pet doors, especially the 'flap-within-flap' design. Instead of a rubber flap, saloon style doors are often made from plastic, acrylic, or plexiglass, and the panels are fitted with weather seal to help keep weather outside.ДЄnother 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. These pet doors usually have a spring or other contrivance to force their closure after the pet has gone through. Some pet doors have side-mounted hinges and swing open like saloon doors. These flaps often feature magnets around the edges to help keep the door closed against weather and wind. 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.

Automatic dog door