Nails with black vertical lines may be traumatic bleeding, nails mother moles, nail fungus, nail band hyperpigmentation, hypoadrenocorticism and so on. 1. Bleeding from trauma: nails are squeezed or smashed, resulting in bleeding under the nails, and the blood coagulates under the nails and there is black bruising. 2. Nail nevus: it is a kind of nevus, manifested as black lines appearing under the nail, and it is recommended to be treated as early as possible because it may turn into melanoma. 3. Onychomycosis: this is a fungal infection of the nails, mostly combined with thickening of the nails, yellowing and graying of the nails, and the appearance of black streaks and lines. 4. Nail band pigmentation: it may be secondary to trauma, local inflammation, etc., and appear black stripes under the nail. Nails with black vertical lines are also seen in other causes, it is recommended to consult a doctor in a timely manner, under the guidance of a professional doctor to standardize the treatment, so as not to delay the condition.