A callus is a thickening of the outer layer of skin, the epidermis. In most cases, the callus is located on the sole of the foot, sometimes on the palm of the hand. The callus is formed mostly following a regular friction or pressure on the skin, often due to bad footwear. The skin thickens in that area for protection. A callus that is too large may cause discomfort when walking. To prevent the formation of calluses, it is advisable to wear comfortable shoes that are a little tight.