Black nails with vertical lines may be related to nail malnutrition, onychomycosis, onychomycosis and so on. 1. Nail malnutrition: when the body lacks vitamin A, vitamin B12 and other nutrients, or nails have a history of trauma, nail quality is damaged, caused by nail malnutrition, may cause nails black with vertical lines. 2. Ringworm of the nails: ringworm of the nails is mainly caused by the infection of dermatophytes, such as red trichophyton, candida and so on. When the dermatophytes invade the nail plate and release various enzymes that decompose the nail plate, thus destroying the nail plate, it can be manifested as black nails with vertical lines. The nail plate of some patients with onychomycosis may also appear turbid, thickening, atrophy, shedding, uneven surface and so on. 3. Nail mother nevus: Nail mother nevus is a kind of pigmented nevus, which can also cause black nails with vertical lines in some patients. It is manifested as evenly distributed pigment strips on the nail plate, and patients need to pay attention to it because of the chance of malignant changes. It is recommended that people with black nails and vertical lines should consult a doctor in time to clarify the cause of the disease and take treatment measures if necessary.