Calluses, also called keratomas or tylomas, are areas of thickened skin caused by repeated friction and pressure. They form to protect the skin and the structures beneath it from injury or damage and can develop on any part of the body.

On the feet, calluses usually develop on the sole (plantar surface), either on the heel or under the metatarsal heads (i.e., the area where the long inner bones of the toes extend into the foot). These areas typically bear most of the pressure from standing and walking. As calluses thicken, additional pressure against the skin may cause pain. Calluses also can form nucleated skin lesions underneath bony prominences on the plantar surface of the foot. These areas also can be very painful.