What is the module of a gear and why is it important?

Module (m) is the ratio of pitch circle diameter to number of teeth (m = D/Z) and represents the size of gear teeth in metric system. It is crucial because two gears must have the same module to mesh properly. Module determines tooth size, strength, and load-carrying capacity. Standard modules (1, 1.5, 2, 2.5, 3, etc.) are preferred for interchangeability and manufacturing economy.