Gear is a mechanical device that transfers rotating or back-and-forth motion and power from one part of a machine to another. Gears vary greatly in size and use. They range from the tiny gears that drive the hands of a watch to the huge gears that turn the propeller of a supertanker. The most common type of gear consists of a metal or plastic wheel or disk with slots called teeth around the edge. Such a gear has an axle in its center. A rack gear is a flat rack with teeth on one side.