Black vertical lines on the nail cover may be caused by nail black lines, onychomatous nevus and malignant melanoma of the extremities. 1. Nail black line: Nail contact with chemicals, acidic and alkaline substances or body lack of calcium and zinc and other nutrients or trauma and collision resulting in bleeding under the nail, etc., manifested as scattered vertical black lines on the nails. 2. Naevus of the nail: The nevus of melanin grows to the root of the nail, resulting in black vertical lines on the nail cover. A few naevus will undergo malignant transformation and become melanoma. 3. Extremity malignant melanoma: black line disease melanoma of nail bed is a common disease in clinic, which mostly manifests black spots on nails and peri-nail skin, with irregular shape and uneven color, and ulcers and other damages may appear. Patients who have the symptom of black vertical line on nail cover should go to the hospital for relevant examination in time, and treat as early as possible so as not to delay the condition.