Vertical black lines on the fingernails may be caused by nevus of the nail or by melanoma, trauma, fungal infection and other diseases. 1. Nail nevus: it is a benign nevus under the fingernail, which will cause black and brown vertical stripes under the fingernail, with clear boundary and regular appearance of the stripes. 2. Melanoma: it belongs to a kind of malignant melanoma, which may also lead to vertical black lines on the fingernail, with wide width, usually with unclear boundaries and fuzzy edges. 3. Trauma: Bruising caused by the rupture of capillaries under the nails due to extrusion, which may also lead to the appearance of vertical black lines on the fingernails. 4. Fungal infection: fungal infection caused by onychomycosis, may also cause nail color changes, manifested as fingernails with vertical black lines, most of which will result in brittle texture, nail thickening and other phenomena. Fingernails have vertical lines, black lines may have other reasons, it is recommended to go to the hospital in time to check, follow the doctor’s instructions for treatment.