Calva is a traditional sport practiced in certain parts of Spain. It has roots going back to pre-Roman times, being developed by the Celtiberians who lived in the modern-day provinces of Ávila, Salamanca, and Zamora. It was a game for shepherds, who threw stones at bull's horns to entertain themselves. With the passing of time, the game was modified: a piece of wood (the calva) came to be substituted for the horn, and the stone was replaced with a cylinder of iron or steel (the marro). The name of calva was derived from the field in which the game came to be played, which was free of obstacles and rocks.