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A trapdoor is a sliding or hinged door, flush with the surface of a floor, roof, or ceiling, or in the stage of a theatre. A hatch, an opening which may also be in a wall and need not be flush with the surface, is similar; in some cases either name is applicable. A small door in a wall, floor or ceiling used to gain access to equipment is called an access hatch or access door.
Originally, trapdoors were sack traps in mills, and allowed the sacks to pass up through the mill while naturally falling back to a closed position.
Many buildings with flat roofs have hatches that provide access to the roof; on ships, hatches—usually not flush, and never called trapdoors—provide access to the deck. Cargo ships, including bulk carriers, have large hatches for access to the holds.
Most 19th- and 20th-century gallows featured a trapdoor, usually with two flaps. The victim was placed at the join. The edge of a trapdoor farthest from the hinge accelerates faster than gravity, so that the prisoner does not hit the flaps but falls freely.
The term trapdoor also refers to a plate in the entry vestibule of a passenger railcar that permits access to high-level platforms when lying flat against the floor of the car, and which can be flipped open to expose steps for accessing ground-level platforms. Many American commuter railroads which operate the Comet railcars made by Bombardier have trapdoors to accommodate passengers boarding and alighting on both high-level and ground-level platforms. Amtrak's Viewliner, Amfleet, and Horizon railcar fleets all have trapdoors.
In theatrical use, "star traps" allowed explosively fast appearances on stage, such as Jinn appearing in a puff of smoke. Unfortunately these devices were also dangerous if the mechanism operated too slowly, causing the actor to fall back through the hole or onto the jagged teeth; thus their use was banned.
Hidden trapdoors occasionally appear in fiction, as entrances to secret passageways, or to secret tunnels. They also appear as literal traps into which a hapless pedestrian may fall if he or she happens to step on one.
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