Law of gearing is satisfied if
(a) two surfaces slide smoothly
(b) common normal at the point of contact passes through the pitch point on the line joining the centres of rotation
(c) number of teeth = P.C.D. / module
(d) addendum is greater than dedendum
I had been asked this question by my school teacher while I was bunking the class.
I want to ask this question from Toothed Gearing topic in section Toothed Gearing & Gear Trains of Machine Kinematics