Black vertical lines on the nail cover may be caused by onychomatous nevus, melanoma and so on. 1. Nail nevus: it is a pigmented nevus located in the nail area, which can be triggered by systemic diseases, trauma, fungal infections, etc. It will cause the nails to have vertical stripes in grayish-black color with regular shape and uniform color. 2. Melanoma: it is a malignant tumor belonging to melanocytes, which may cause nails with black vertical stripes, irregular shape of color stripes, uneven color, and may be accompanied by ulceration, bleeding and other damages. Nail cover has black vertical lines, there may be other reasons, need to go to the hospital in time to check, clear cause of disease and follow the doctor’s instructions for treatment.