Relates to two classes of spur master gears for use as reference and inspection standards with pitches in the range 2 to 64 normal diametrial pich inclusive. The two classes are designated A and B. Class A master gears are intended to test precision class product gears and Class B to test gears for general purposes. This specification may also be applied to helical master gears in so far as tolerances are concerned, but the tolerances given have been chiefly derived from information from spur master gears.