The number of nerves in the root of a tooth is determined by the number of roots of the tooth, usually one root corresponds to one nerve, i.e., a single-rooted tooth has one nerve, and a multi-rooted tooth has 2~3 nerves, as follows. The mesial incisors, lateral incisors, cuspids, maxillary second premolar, mandibular first premolar, and mandibular second premolar are single-rooted teeth, and their roots usually have one nerve. The maxillary first premolar, mandibular first molar, and mandibular second molar are usually double-rooted, and their roots are usually two nerves. The maxillary first molar and maxillary second molar have three roots, and their roots are usually three nerves. The third molar is more variable and has a variable number of roots, so the number of nerves in its root is also variable.