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Games intended to be run under Fidonet-era bulletin board software. They're called "doors" because they were separate programs that would run on the BBS host. Since machines didn't necessarily have multitasking capabilities, the main BBS program would "shell out" and start up the door program, which would then communicate with the user. When the door exited, the BBS software would start back up. Hence a door -- an excursion from the BBS to the outside world.