Black horizontal stripes on nails can be caused by nevus of onychomycosis, nail dystrophy, nail fungus disease, trauma, heredity and other factors. 1. Nevus of Nail: Nevus of Nail is a kind of tumor originated from the nail mother cell, mainly single, a few may appear multiple cases, which can be manifested as black stripes on the nail plate, and the stripes will gradually widen with the progression of the disease, and the color may also become darker. 2. Nail dystrophy: Nail dystrophy can lead to damage to the structure and function of the nail thumb, which in turn leads to the formation of the nail plate obstacles to the appearance of the shape, color and texture of the changes that can occur in the black horizontal stripe changes. 3. Nail fungal disease: nail infection with fungi and other pathogens can lead to the emergence of nail fungal disease, which in turn causes changes in the color and texture of the nail plate, which can be manifested as black horizontal lines. 4. Trauma: after trauma to the nail plate or nail bed, due to localized bruising, black horizontal stripe-like changes can occur. 5. Heredity: it may be caused by congenital genetic factors, and it can be clarified whether there is a family genetic history of this kind of disease. When there are black horizontal stripes on the nails, you should consult a doctor in time, after a clear diagnosis, under the guidance of a specialist to actively treat the disease.