The black or brown straight line visible on the nail layer is called melanin in medical language. This line can be from light to dark in color and can be seen in any nail of the hand or foot. Sometimes it is present from birth and sometimes it appears with age.

Often black line in nails is not due to any serious disease, there can be many common reasons behind it, like injury to nail, lack of nutrition in the body, effect of some medicines, hormonal changes. This is more common in people with dark skin. For this reason many people do not take it seriously.