A rack is a gear whose pitch line is a straight line instead of a circle. Fig. 116 shows an epicycloidal rack in gear with a 16-tooth pinion. The describing circles are of the same size in the figure, although they might be of different sizes. The teeth on the pinion are drawn as described for Fig. 114, the construction lines for drawing them not being shown. The curves which form the faces and flanks of the rack are cycloids. The describing circle C, rolling on the pitch line of the rack, generates the cycloid PF, which forms the flanks of the rack teeth; and the describing circle D, rolling on the pitch line of the rack, generates the cycloid PH, which forms the faces of the rack teeth. The addendum and root lines are drawn parallel to the pitch line, and at a distance from it equal, respectively, to the distances A and R of the pinion. The teeth are spaced off on the pitch line of the rack by laying off the distances PK, KL, etc., equal to the circular pitch of the pinion.