Pulleys and gears are mechanical devices that change the direction or type of applied effort or motion. A single pulley reverses the direction of effort, while crossed belts turn pulleys in opposite directions. Gears can be arranged in trains to change the direction of rotation, with idler gears introducing intermediary rotations. Special gears like bevel gears, worm gears, and rack and pinions are used to change the angle or type of motion in devices like hand drills, guitar tuners, and car steering systems.