A sprocket is a profiled wheel with teeth that meshes with a chain, track or other perforated or indented material. It is distinguished from a gear in that sprockets are never meshed together directly, and differs from a pulley in that sprockets have teeth and pulleys are smooth. Sprockets are widely used in bicycles, motorcycles, cars, tanks, and other machinery. Sprockets typically do not have a flange. Some sprockets used with timing belts have flanges to keep the timing belt centered.