A nail mother nevus is a type of mole that grows at the base of the nail, and as the nail grows, the nevus cells bring out melanin to form a black line. Both fingernails and toenails can have nail moles, which mostly grow on fingernails and usually appear alone without itching or pain. The nail mother is the place where the nail grows. If the nail mother melanocytes increase and the nail plate grows with it, a black line, called a nail black line, will appear and is the most common symptom of nail mother nevus. A nail black line is not necessarily a nail mother nevus, but also needs to be distinguished from nail melanocyte activation, nail nigra, nail fungus disease, nail malignant melanoma, etc. Therefore, when a nail black line appears, a specialist needs to do a comprehensive diagnosis such as dermoscopy and pathological examination. If regular nail black lines appear, there is usually not much to worry about, but it is important to watch regularly for suspicious changes. If abnormal changes such as widening, unevenness, sudden deepening of color, unclear borders and skin ulcers appear in a short time, you should consider whether they are malignant and need to go to the dermatology department of a hospital in time for a comprehensive assessment and treatment by a specialist. There is no obvious correlation between the appearance of nail nevus and diet, so a normal diet is sufficient, and it usually grows spontaneously without any effective prevention methods.