Spur gears are used to transmit rotary motion between parallel shafts. They are cylindrical, and the teeth are straight and parallel to the axis of rotation. The pinion is the smaller of two mating gears; the larger is called the gear or the wheel. The pitch circle, B in Fig. 33.1, is a theoretical circle upon which all calculations are based. The operating pitch circles of a pair of gears in mesh are tangent to each other.