A callus is a thick, hardened area of skin. They form when the skin is subject to constant friction, pressure or other irritation. They are quite commonly found on the feet, but can be found frequently on the hands of people who frequently use tools, such as carpenters, but can even form in small areas of people who frequently use writing instruments like pens and pencils, or who play string instruments like the guitar or violin. . A callus is only medically significant in a diagnosis where there is no evidence of repetitive wear. For example, both arsenic and syphillis can result in calluses.