In English-speaking countries, instead of the module , often the diametral pitch P_d = z/d = 1/m_t is used. For the diametral pitch in the normal plane P_nd the following identities hold: P_nd = 25.4/m_n = P_d/cos(β), with m_n in [mm] and P_d in [inch-1], and the helix angle β.

The module is among the most important gear dimensions for spur- and helical gears. The module represents the distance between the teeth, which is also referred to as the pitch. The pitch is the distance between two consecutive right or left flanks on the pitch circle (see figure). Even if the reference diameter d is a rational number, the pitch of the teeth in the circumferential direction is an irrational number (due to multiplication with π). Therefore, unlike in English-speaking countries, it is not the pitch (=d/z * π) that has become established as the standard parameter, but the module (=d/z). For helical gears (β≠0), a distinction must be made between the module in the transverse plane m_t and the module in the normal plane m_n. The module can also be regarded as a measure of the size of the gear's teeth, which is why gears with large or small teeth are also referred to as large-module or small-module gears. The tooth height is usually specified in relation to the module.