Teeth with black lines may be related to pigmentation or caries. 1. Pigmentation: If the patient smokes, drinks tea, cola and other foods containing more pigment for a long time, dark-colored attachments may form under the surface of the teeth. If the daily brushing is not careful enough to clean these attachments in time, the pigment may gradually deposit on the teeth and become difficult to remove, showing black lines on the teeth. 2. Caries: The cross-section of the teeth is not flat, there will be some small grooves. If the brushing posture is not correct, it is difficult to brush the teeth clean in all directions, in this case, the surface of the teeth on the dirt will breed a lot of bacteria, the formation of caries, which may be manifested as black lines on the teeth. It is recommended to go to the hospital in time for examination and timely treatment such as cleaning and restoration to avoid deterioration of teeth with black lines.