<P> In mechanical engineering, a key is a machine element used to connect a rotating machine element to a shaft . The key prevents relative rotation between the two parts and may enable torque transmission . For a key to function, the shaft and rotating machine element must have a keyway and a keyseat, which is a slot and pocket in which the key fits . The whole system is called a keyed joint . A keyed joint may allow relative axial movement between the parts . </P> <P> Commonly keyed components include gears, pulleys, couplings, and washers . </P>

The groove in a shaft for a key is called a