Rack is a kind of special gear whose teeth are distributed on strip. It can be divided into two kinds of rack, spur rack and helical rack. Spur rack can be used together with spur gear and helical rack can use together with  helical cylindrical gears. Teeth shape of rack should be straight not involute (For the tooth surface, it is flat), it means that radius of reference circle is the same as infinite cylinder gear. Since the teeth shape of the rack is straight, the pressure angle of each point on the teeth shape will be the same and it equals to the inclination angle of the teeth shape. This angel is called tooth angle and the standard value is 20°, the straight line that parallel with the top line of the teeth has the same pitch and module. The straight line that parallel with the top line of the teeth and the teeth thickness is the same as the space width is called graduation line( center line), which is datum line for calculating rack dimensions.