Common causes of a light brown vertical line on the thumbnail include onychomatous nevus and onychomelanoma. Usually, if it is a single light brown vertical line with regular morphology and uniform pigmentation, it is usually considered as a nevus of the nail. 1. Onychomatous nevus: this disease is a kind of pigmented nevus growing on the nail area, which can be induced by systemic pigmented diseases, acanthosis nigricans, subnail hemorrhage, fungal infection and other factors. It is usually manifested as brown or black-brown strip under the nail plate, with regular boundary and uniform color. 2. Nail melanoma: the cause of this disease is not clear, at present, it is believed that it is mainly related to genetic factors and ultraviolet radiation, etc. Patients with nail melanoma usually have a family history of melanoma. If the original nail black line color change, morphology change, accompanied by ulceration and bleeding and other changes, you need to consult a doctor in time, after surgical resection to improve the pathological examination. It is recommended that patients go to the dermatology department of regular hospitals according to their own conditions, follow the doctor’s instructions to clarify the diagnosis and reasonable treatment, so as not to delay the condition.