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This door was predicted to be a back or interior door of a middle class home. Cats at the time were mostly just for catching mice but there were some people that would give their cats a bit more freedom with a cat hole and people also used these cat holes to peep inside for if their knocks were unanswered, very few of these doors with cat holes survived this period.

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This is from Chetham’s Library, UK’s oldest public library, first built in 1421 as a college and then used as a library from 1653. In this photo is one of the original doors and you can see at the bottom is a cat flap, it was added from when they made the building a library, and they added it because in the building there was a problem with mice chewing away at the booking, destroying valuable knowledge, so a cat was the most effective solution to the destruction - in combination with a cat flap, to give the predator free access to the premises.

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These cat flaps are completely made of plastic, with an lock mechanism at located at the bottom which is very easy to use. These basic cat flaps have been around for maybe 50 years and they are still manufactured to this day. They are cheap and easy to use.

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This cat flap is very interesting as the edge of it is metal and then the flap made using a bendy plastic with magnets at the bottom for it to stick to the metal plate at the bottom. This cat flap has been produced by the company Pet Doors ever since around 2015

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Here is a simple and cheap transparent cat door, but the problem with ones just like this is that they are normally very weak, as the plastic can be easily cracked or broken apart so I would not recommend going with a design like this.

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In 2008 the company SureFlap released the first microchip cat flap, inside of the door located in the top part is a device that will automatically detect if on of the selected cat chips, that you have assigned to the door, is directly below it and it will unlock the flap. This is very useful nowadays as it will not allow other cats to enter through your cat flap but every now on then you will have to put new batteries inside of it or it will not work properly and most cats now will have chips inside of them to provide a form of identification if ever needed in situations.

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Here is a fun little design for a indoor cat hole, these would go possibly just at the bottom of interior doors inside your home but of course you could put it anywhere. These are very cheap and made entirely of plastic.

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This design of cat hole is very interesting as you can see it has a brush around the inner part which will slightly clean your cat as it walks through it, the materials in this are mainly wood and plastic with its outer door shape made of wood and its little plastic brushes connected to a plastic sheet using little plastic rings. This type of cat hole is mostly only placed inside of interior doors as it would leave an opening for wind and insects to enter through its big open hole.