A callus is an area of thickened skin that forms as a response to repeated friction, pressure, or other irritation. Since repeated contact is required, calluses are most often found on the feet and hands, but they may occur anywhere on the skin. Some degree of callus, such as on the bottom of the foot, is normal. Although calluses can occur anywhere on the body as a reaction to moderate, constant "grinding" pressure, they are most often found on the foot (where the most pressure and friction are applied). On the feet, arguably the source of the most problematic calluses, they typically form on the metatarsal-phalangeal joint area ("balls of the foot"), heels and small toes due to the compression applied by tightly fitting shoes.