Abstract
This paper shows that the progressive direction of tooth meshing is the direction of relative velocity between meshing gears. The progressive direction of meshing has been neglected in gear kinematics until now, so that gear manufacturers could not get the standard pattern on the tooth bearing of gears, and gear engineers could not know the real meaning of tooth meshing. We understand loosely the tooth meshing which is the most fundamental phenomenon in gears. The tooth meshing means the sliding contact of a pair of slide lines.
