Black vertical lines on the thumbnail are generally considered to be a nevus of the nail. This condition usually depends on the age of the patient, if it is relatively young, observation is the main concern. Because there is a greater risk of malignant transformation of onychomatous nevus, which is also called melanoma malignant transformation, if the black lines do not become more colorful and clearer during observation, it should not be handled. If it is older and the black line has worsened and thickened, it is recommended that it should be treated surgically by pulling out the nail and removing the onychomatous nevus. This is also a relatively simple and common surgery, and the main purpose of the surgery is also to prevent malignant changes of the nail mole.