Conjugate Action 

To be able to transmit a uniform rotary motion from one shaft to another, the peripheral speeds at the pitch circles of the two gears must be identical. Referring to Figure 5, one can imagine the two gears as two cylinders with diameters the same as the base circle, connected together by means of a steel band along the line of action.
Figure 5 Conjugate motion.
A single point of this steel band would generate an involute from each of the two base circles and as the same point generates both involutes, it is clear that the involute tooth profile would duplicate the movement of the two base circles at exactly the same peripheral speed. It is important to note that any variations from the true involute tooth shape will introduce varying circumferential speeds between the teeth and consequently generate vibration, noise and other problems. The angle that this line of action forms with a tangent to the two pitch circles is the pressure angle.