Many people have noticed the small crescent-shaped white area at the base of their fingernails without ever knowing what it actually is. It has a name—the lunula—and despite looking like a simple white mark on the nail, it is a visible part of the nail’s growth system.

The lunula is most noticeable on the thumbs and may appear smaller or even seem completely absent on some of the other fingers. Its pale appearance comes partly from the structure of the developing nail in that area, which makes the underlying blood vessels less visible than they are through the pinker portion of the nail.

What surprises many people is that the visible crescent represents only part of what’s happening beneath the skin. The nail matrix, located largely underneath the skin at the base of the nail, produces the cells that eventually become the hard nail plate. As new cells form, older ones are pushed forward, gradually creating the fingernail we see.

The size and visibility of the lunula can naturally vary considerably from person to person and even between different fingers on the same hand. Having large crescents doesn’t automatically mean someone is healthier, and barely visible lunulae aren’t automatically evidence that something is wrong. Nail appearance can be influenced by normal anatomy as well as many other factors.

So that mysterious white half-moon isn’t a useless marking or a hidden personality indicator. It’s called the lunula, and it is associated with the nail-forming region responsible for producing the nail as it grows. It’s a tiny piece of human anatomy that many of us look at every day without ever realizing what we’re actually seeing.