In fact, the gear teeth are not located on a simple linear rack. The circular spline teeth are located on a circle and the flexspline teeth are located on an oval surface formed by the wave generator. These two curved surfaces cause an inclination angle change between a tooth on the flexspline relative to the circular spline as the tooth moves into the engagement from the minor axis to the major axis. Such inclination angle is ignored when it is assumed that the circular spline and the flexspline are straight racks.