What are the dark stripes that run down the nail bed of fingers and toes called? what causes them?

Be careful: If you have dark skin it's fairly common to find dark stripes running down your nails this occurs in more than 77% of black people over the age of 20 and in most cases is perfectly normal however it is not something to ignore dark stripes may sometimes be a form of skin cancer that affects the nail bed called subungual melanoma it's therefore important that your doctor checks it to rule melanom.