Black vertical lines on the fingers are usually caused by various factors such as nevus of the nail, extrusion and melanoma of the nail. 1. Nail nevus: the cause is not very clear, it may be related to active melanin proliferation, causing black vertical lines on the nails, which is a typical symptom. 2. Extrusion: patients are hit by heavy objects or squeezed by the door, resulting in capillary hemorrhage, causing nails to appear black vertical lines. 3. Nail melanoma: mainly due to trauma, infection, physical and chemical factors, resulting in active melanocyte proliferation, secretion of more melanin or melanoma cell proliferation into a mass, resulting in black vertical lines on the patient’s nails. There may also be other reasons for the black vertical lines on the fingernails, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or treatment.