Action Of a Pair Of Mating Involute Teeth Let a and b be the base circles of a pair of mating involute gears. Line CD is a common tangent to the base circles. while b is the line of centers assume that CD is part of a string being unwound from A and wound upon B, while A and B rotate together in such a manner that string CD remains taut at all times. When A and B start to rotate, point C on the string will leave