This summer, Charmaine Sherlock noticed a thin black line under her nail of her left thumb — what looked like a splinter growing from her cuticle. She thought perhaps she was having an ...

Charmaine Sherlock, 34, first spotted a thin, black line beneath the cuticle of her left thumb in the summer, and thought she was having an autoimmune reaction, she told TODAY.com. A nurse ...

Most of the time, thin, vertical black lines under your nails are no cause for concern. But if you've had them for a while or you just want to make sure they aren't a sign of something more ...