A vertical dark line, we call it as longitudinal melanonychia. A longitudinal melanonychia is very common in dark skinned people and doesn't indicate that you have melanoma. In fair skinned people, I don't mean Indians, I mean Caucasians and Europeans, and you have a longitudinal black line on your nail, then you do have to be concerned about melanoma. The characters of longitudinal line due to melanoma are slightly different from other causes. Other causes are common congenital cause like peopl