The basic rack profile is the normal section through the teeth of a basic rack which corresponds to an external gear with number of teeth z = ¥ and diameter d = x. The tooth of the basic rack profile is bounded by the tip line at the top and by the parallel root line at the bottom. The fillet between the straight tooth flank and the root line is usually of circular arc form.

The basic rack profile is fundamental to the specification of involute gears. It determines the tooth profile on the gear, the generating rack profile and associated rack shaped tools. The relationship between these can be seen in Fig.1.
As with the standard rack profile, the tooth thickness is equal to the tooth space width at the profile datum line and hence equal to half the pitch, gear and mating gear can be cut with the same tool. The entire dimensions for defining the basic rack profile must be contained in the tooth data.