Applies to external and internal involute spur and helical gears for use with parallel shafts and having modules not less than 1 mm. The basic rack and tooth forma are specified together with requirements for terminology and notation, first and second preference modules and fundamental tolerances which determine the grades of gear accuracy. Twelve grades of accuracy are associated with each gear element and thus the standard can be applied to meet requirements ranging from coarse commercial applications to precision gears for high-speed and/or high-load applications.