Clocks and watches are usually distinguished by the size of the inner machine. According to international practice, the center diameter of more than 50 mm, thickness of more than 12 mm for the clock; Diameter 37 ~ 50 mm, thickness of 4 ~ 6 mm, called pocket watch; Diameter below 37 mm for watches;

Diameter is not greater than 20 mm or machine center area is not greater than 314 square millimeters, known as the female table. The watch is one of the smaller, stronger and more precise machines invented by man.