When you look in the mirror, are you bothered by the appearance of extra fat underneath your chin? Perhaps you choose to wear high-collared shirts or turtlenecks to try to hide this area. If so, you may have a double chin. A double chin, also known as submental fat, is a common condition that occurs when a layer of fat forms below the chin. It can be associated with weight gain, though you do not have to be overweight to have a double chin. Lax skin and genetics can also play a role in the development of a double chin.