Rack gears have their teeth cut on a flat surface. As there is no curvature on the pitch line, rack teeth cut to a basic involute tooth form have straight sides. The teeth may be cut at right angles to the direction of travel as for a spur gear, or inclined to mesh with a single helical gear. Torque can be converted to linear force by meshing a rack gear with a pinion (external spur gear or external single helical gear), when the pinion turns the rack moves in a straight line.  Conversely, if the rack is oscillating back and forth then the pinion will rotate clockwise then anticlockwise.