There are many reasons for having a black line on the neck, such as hyperpigmentation, acanthosis nigricans, and inadequate cleansing. 1. Hyperpigmentation: Neck belongs to the exposed skin, if subjected to long time ultraviolet radiation, it may lead to hyperpigmentation of the neck skin, so there will be the phenomenon of a black line on the neck. 2. Acanthosis nigricans: Acanthosis nigricans can also lead to a black line on the neck, the disease will also be accompanied by papillary hyperplasia, which may be related to heredity, obesity, diabetes, tumors and other conditions. 3. Cleaning is not in place: a black line on the neck may also be the result of skin cleaning is not in place, skin cleaning is not in place, can lead to the accumulation of local dirt, so there will be the phenomenon of black lines, usually after cleaning will be removed. There are many reasons why there is a strip of black lines on the neck, if you exclude the cleaning factors, it is recommended that patients go to the hospital in a timely manner to improve the examination, under the guidance of the doctor to take standardized treatment, not to ignore the symptoms, to take it lightly.