A rack and pinion is a device which converts rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack". As the pinion rotates it causes the rack to move relative to it, thereby translating the rotational motion of the pinion into linear motion of the rack.

As woodworkers we will be most familiar with their use in machinery, however they are widely used in other industries as well, automotive steering, pneumatic actuators, and railways.

Rack railways are particularily interesting as they are used to help a train up a steep incline but just importantly as a brake on decent.