Possible causes of black lines in nails include naevus melanocytic nevus, naevus melanoma, and subnail bruising. 1. Melanocytic nevus of the nail: Melanocytic nevus of the nail is a common cause of black lines in the nails, which mostly occurs in childhood, especially in the thumb, and most patients have obvious pigmentation in the inner strips of the nails, and it is often accompanied by hyperpigmentation of the proximal nail folds and perinail tissues. 2. Nail melanoma: a few patients have black lines in nails which may be caused by nail melanoma, the color of black lines in nails of this kind of patients is uneven, irregular edges, and the color band is wider than the distal one at the proximal end of the nails, which may be accompanied by nail ulceration, nail loss, nail destruction, nail peeling and other symptoms, and the skin around the root of the nails or the side nail wall of the patients may become black, the condition of nail melanoma is progressing rapidly, and it needs to be treated in time. 3. Subnail bruise: when the nail is squeezed, impacted and other violent damages, it will lead to the rupture of capillaries in nail bed, followed by subnail bruise, which will cause the occurrence of the phenomenon of black line in the nail, and these nail traumas may also involve the nail plate, proximal nail folds and other parts of the nail, which will lead to the damage of nail matrix and secondary nail plate abnormality, and cause the occurrence of longitudinal black nails. It is recommended that patients with black lines in the nails go to the hospital to clarify the cause of the disease, timely treatment, to avoid further aggravation of the condition.