A specified amount of time in which a person, called an apprentice, becomes bound by legal agreement to work for another, his master, in return for instruction in an art, a craft, or a business. Once an apprentice completes his term of apprenticeship he would be called a journeyman. He would become a master in his turn only when recognized masters judge an example of his work as worthy to have been made by a master — a masterpiece. The apprenticeship system was common across Europe when guilds were strong in the Middle Ages until the arrival of the Industrial Revolution (about 1775-1875). Leonardo da Vinci (Italian, 1452-1519), for example, was apprenticed to Andrea del Verrocchio (Italian, 1435-1488), in whose work art historians believe they see young Leonardo's work appear and mature. This system has been replaced almost completely by formal schooling and the free-market's gallery system.