Turnstile
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A turnstile is a form of gate which allows movement of people into a building or venue, but not out. As such, a turnstile is often used for controlling the input of paying people, for an event or in public transport, for example. Turnstiles are also used for counting the numbers of people passing through a gate, even where payment is not involved.
Turnstiles use ratchet mechanisms to allow the rotation of the stile in one direction allowing input but preventing rotation in the other direction.
Mechanical turnstiles are lesser used these days, with electronic gate and ticketing systems becoming more common.
In the first half of the twentieth century, it was common for entry to public lavatories in Britain to be controlled by turnstiles.
The symbol <math>\vdash<math> has taken the name turnstile in mathematical logic because of its resemblance to a typical turnstile if viewed from above.