Crowding is classified as mild, moderate or severe depending on the degree. The difference between the length and curvature of the arch and the existing arch is called dental crowding, which is classified as mild, moderate or severe according to the degree. Mild is 2-4mm, moderate is 4-8mm, and if the degree of crowding reaches 8mm, it is called severe crowding. In clinical practice, the orthodontist will decide whether orthodontics needs to extract teeth according to the degree of crowding, and whether orthodontics needs to extract teeth is a more professional issue.