When there is excessive pressure or friction on the skin, calluses and corns form to protect the skin and the structures beneath it. Calluses on the feet can develop into corns.
Calluses usually develop on the soles of the feet, on the parts that bear most of the pressure when you walk and stand. A callus, when it thickens, can cause added pressure inside a shoe against the skin.
Walking barefoot can cause thick calluses under the soles, but if someone develops calluses even if they wear shoes most of the time, it is a sign that there is an underlying problem (such as bunions or flat feet) which is causing pressure between the skin and the bone underneath it.
Shoes with high heels or narrow toes can also cause calluses – the former because the shape of the shoe shifts the body weight onto the front metatarsal bones and the latter because the narrow shape of the shoe leave little space for the toes, which are then squeezed together, causing friction.
South African Podiatry Association (SAPA)