Short for tooth, it refers to each protruding part on a gear that is used for meshing. These protruding parts are generally arranged in a radial shape, and the teeth on the paired gear come into contact with each other, allowing the gear to continuously mesh and operate. Closed gear transmission generally has a higher rotational speed. In order to improve the smoothness of the transmission and reduce impact vibration, it is better to have more teeth, and the number of teeth for the small gear can be taken as z1=20-40. Open (semi open) gear transmission, due to the main wear failure of the teeth, in order to prevent the gear from being too small, it is not advisable to choose too many teeth for the small gear. Generally, z1=17~20 is recommended. When two gears mesh, the tooth top of one gear always enters the tooth root of the other gear. In order to prevent thermal expansion and space for storing lubricating oil, the tooth root height is required to be greater than the tooth top height. To this end, the tooth top height coefficient and top clearance coefficient have been introduced.