Byzantine or Russian emperor. The title, derived from caesar, was used in the Middle Ages to refer to a supreme ruler, particularly the Byzantine emperor. With the fall of the Byzantine Empire in 1453, the Russian monarch became the only remaining Orthodox monarch, and the Russian Orthodox clergy considered him a possible new supreme head of Orthodox Christianity. Ivan IV (the Terrible) was the first to be crowned tsar, in 1547. Though theoretically wielding absolute power, he and his successors were limited by the power of the Orthodox church, the Boyar Council, and the successive legal codes of 1497, 1550, and 1649. In 1721 Peter I changed his title to Emperor of All Russia, but he and his successors continued to be popularly called tsars.