Thumb nail cover has vertical black common causes breast as follows: First, the most common clinical nail mother nevus, and nail trauma caused by nail black line. The nail mother nevus is a pigmented nevus at the root of the nail. The melanin particles of the pigmented nevus will gradually form a black line on the nail as the nail is synthesized. If the nail becomes thicker or darker within a short period of time, it needs to be surgically removed. The specific procedure is to remove the nail cap from the nail root, surgically dig out the pigmented nail, and then stitch it up. Secondly, nail trauma causes nail black line, nail trauma may cause nail root dot bleeding, blood will form a relatively short black line with the growth of nail cap, this black line will gradually move out of the nail with the nail cap, no special treatment is needed.