The rack can be designed as a spur gear with an infinitely large diameter. The pairing of a gear rack with a spur gear is referred to as a rack and pinion gear. The movement of the rack is rectilinear and limited by its finite length. The spur gear is the most commonly used gear. A cylindrical disk is toothed on its circumference. If the axes of a spur gear and its mating gear (spur gear or spur-toothed shaft) are parallel, the result is a spur gear.