During a person’s life, there are two sets of natural teeth in the normal state, which can be divided into milk teeth and permanent teeth depending on the time of eruption. Generally, milk teeth start to erupt from about 6 months after birth, and permanent teeth start to erupt around 6 years old, the specific eruption time varies from person to person, and there are differences in the names of teeth. 1. Milk teeth: There are 20 milk teeth, commonly the upper and lower pairs are neatly arranged, and there are 5 on each side. The milk teeth start from the midline towards the deepest part of the mouth and are named as milk central incisor, milk lateral incisor, milk cusp, first milk molar and second milk molar. 2. Permanent teeth: The number of permanent teeth is 28-32, and the maxilla and mandible are divided into left and right sides with 7-8 teeth each. Starting from the midline toward the depth of the mouth, the names are central incisor, lateral incisor, cuspids, first premolar, second premolar, first molar, second molar, and third molar, where the third molar is also called wisdom tooth. In addition, the number of permanent teeth is not fixed, and some people may be accompanied by extra teeth or born with missing tooth germ resulting in fewer teeth, such as malformed extra teeth between maxillary central incisors, or unerupted third molars. In addition, the eruption of the third molar is often difficult or obstructed due to the position of the remaining alveolar bone and the narrower contour of the bone arch.