The bottom of the tooth should refer to the cervical part of the tooth, because the root of the tooth is covered under the gum and usually cannot be seen. Therefore, the black color on the neck of teeth is commonly caused by two kinds of cases: one is caused by excessive calculus and plaque; the other is caused by tooth decay. It is recommended to go to the stomatology department of the hospital for a detailed examination to distinguish which is the case. If the blackening is caused by excessive calculus and plaque, it is recommended to visit the periodontal department of the hospital and have the teeth scaled to remove the calculus and plaque to avoid repeatedly stimulating the gums and causing gingivitis. If the blackness is caused by dental caries, it is recommended to consult the endodontics department of the hospital to remove the decaying matter and fill the treatment, so as to avoid the aggravation of dental caries and the occurrence of pulpitis.