Note: thicker lines that create a horizontal stripe on the nail are ... Most of the time, thin, vertical black lines under ...

and the thickness can range from resembling a thin pencil mark to a broader, more noticeable band. A black line can also ...

A small black line that forms vertically underneath your nail may be a splinter hemorrhage, while horizontal lines may be melanonychia. These may be harmless or a sign of a more serious health ...