The Switch (sexuality) reference article from the English Wikipedia on 24-Jul-2004
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Switch (sexuality)

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In the field of sexuality, a switch is someone who can change from one sexual role to another.

In anal sex among men, a switch is someone who participates both in the penetrative and penetrated roles at different times.

In BDSM, a switch is someone who participates in BDSM activities as both a top and a bottom. Switches are very common; partners may switch roles based on mood, desire, or being with a different partner. They might also switch because both partners in a relationship are (for instance) bottoms, and they will take turns at 'switching' to top so that they can continue their activities. A person who engages in self-bondage can be viewed as taking both roles simultaneously.