The working principle of the internal parts of an automatic mechanical watch, and the classification of automatic watches. Automatic watches are divided into two types: fully automatic and semi-automatic. In a fully automatic watch, the rotor can wind the mainspring when rotating in any direction, whereas in a semi-automatic watch, the rotor only winds the mainspring when rotating in a specific direction. Another explanation is that if the rotor can rotate fully around its axis, it is considered fully automatic; if the rotor can only rotate 270 degrees, it is considered semi-automatic. Due to the inefficiency of the materials used in this type of automatic mechanical watch, they have been phased out. Nowadays, the vast majority of automatic watches belong to the fully automatic category.