In this mechanism, for every turn of the driver wheel A, the driven wheel B makes a quarter turn. The pin, attached to driver wheel A, moves in the slots causing the motion of wheel B. The contact between the lower part of driver A with the corresponding hollow part of wheel B, retains it in position when the pin is out of the slot. Wheel A is cut away near the pin as shown, in order to provide clearance for wheel B as it moves. If one of the slots is closed, A can make less than one revolution in either direction before the pin strikes the closed slot, stopping the motion. Early watches, music boxes, etc., used Geneva wheels to prevent over winding. From this application, they also are called Geneva Stops. As a stop, wheel A is fastened to the spring shaft, and B turns on the axis of the spring barrel. The number of slots in B depends upon the number of times the spring shaft should be turned.