As told in a Monster World timeline created by Pixar artist Bud Luckey during pre-production of the original film that was subsequently published in the Monsters, Inc. Employee Handbook tie-in, door technology was invented around 1800 to improve travel between the monster world and human world by traveling directly to kid's rooms. As door technology grew more popular, a steam-powered door track system was invented in 1850 to allow for new scaring companies to easily organize and store the doors, with a fully automated system being created in 1920 just in time for scream energy to become the main Monster power source.
If a door leads to the bedroom of a child that is unable to be scared anymore (such as the red door that led to the bedroom of a 6 year-old girl whom Claws Ward failed to scare and almost got touched by), then it is declared "dead" and is immediately destroyed using a shredder, leaving only the doorknobs intact (though it is revealed at the end of the first film that the door's pieces can still be put back together and therefore repairing it with time). Doors can also be destroyed if dropped from a great height (such as the one Randall dropped from a conveyor belt in the door vault, prompting Mike and Sulley to enter it immediately before it is smashed, and the door leading to the Trailer Son and Mom's trailer and hence Randall's defeat) or overloaded with an unusually strong scream (such as the door leading to the cabin as a result of Sulley and Mike scaring several human adults).
In Monsters University, due to it being a prequel, all of the doors' stations were much larger, more rectangular, and clunkier in appearance, probably so that they all appear very old and outdated compared to the original film. It should also be noted that all of the doors in both films are made of wood, except for the one used for banishing monsters to the Human World in the Himalayas, which is made of metal.
As most kids know, a closet door is one of the few things capable of keeping monsters at bay. With this simple dividing line, the Pixar team created worlds for both sides of the door. Each side had its own rules, the most important of which was forbidding anything from the human world to cross into the monster world.
Pixar created all of the doors in the door vault from combinations of twenty-six paint colors, twelve styles, eight wood colors, ten decals, six door knobs and three hardware types. This can create a maximum total of 449,280 possible combinations.
Even the shredders that are used when doors become "dead" can malfunction, as seen in a blooper.
They are not to be confused with normal doors that are only used to access adjacent rooms.
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