Relative movement of members (10,11) of a lock is normally obstructed by an element (14) which can be moved by a solenoid (18) entirely into a recess in one of the members so that the one (11) and the obstructing element (14) can turn together relative to the other member (10) and solenoid. The arrangement may be used to prevent turning of a key-receiving member relative to a housing or as a clutch between two members mounted rotatably in a housing.