A serrated shaft is a series of adjacent triangular projections which are named teeth and are formed around its periphery. The theoretical major diameter is that of a circle passing through the apices of the fundamental triangles formed by the flanks of the teeth, the theoretical minor diameter is that of a circle passing through the points where the bases of the fundamental triangles touch.
The effective diameter of the serrated shaft is that of a circle midway between the theoretical major and minor diameters. Similarly a serrated hole comprises of a series of adjacent triangular teeth from which fundamental triangles are formed. Serrations provide a means of transferring drive from one member to another with the stress around the join evenly distributed.