There are two cases of black lines on fingernails, 1. nail mother nevus: nail mother nevus is similar to a black mole growing on the skin, which is a benign skin manifestation, but some patients may be malignant rash with black lines on the nails as the initial manifestation, so changes need to be observed, such as growing faster, uneven color, unclear border or vesicles, exudation, rupture and bleeding, which may suggest malignant tendency and need to timely treatment by a doctor. If the rash does not change much and grows slowly, it can be observed regularly and there is no need to worry about it, the possibility of benign is higher. 2. sub nail petechiae: sub nail petechiae and petechiae are mostly rashes that appear after the nail is subjected to external force or after the nail is exerted, in this case the rash will gradually fade on its own, there is no need to worry too much, you can improve the blood routine and coagulation function examination to check for thrombocytopenia The rash will gradually subside on its own, and there is no need to worry about it.