Key stock is a small metal block or wedge inserted between a shaft and the hub of a pulley, gear, sprocket, or other rotating power device. It provides driving torque and prevents movement. One part of the key fits into a precut slot in the shaft known as a key seat. The remainder slides into a groove in the hub called a keyway. Woodruff keys are flat, semicircular keys that rock into place and adjust to an angular spine without coming unseated. Machine keys are elongated, with either rounded ends, square ends, or tapered gib heads. Step keys, also known as offset keys, are precision-milled with steps notched at regular intervals for repairing sprockets, gears, and pulleys.