Where are calluses most commonly found?

Hands, feet: Calluses are a response to repeated pressure and rubbing and are a protective measure by the body. The hands and feet are the sites where these are common.

Hands and feet: Areas of pressure and friction.

On the skin.: Not to put too fine a point on it but, they are usually found and formed between a bone and a hard place. Any skin trapped between an underlying bone and an externally applied pressure is apt to form a callus. That series of circumstances just occurs most of ten on the hands and feet.