Leavened vs. Unleavened Eucharistic Bread

This concept concerns whether the bread consecrated at the Eucharist is leavened or unleavened — a question that sharpened East-West tensions around the year 1054, though the practices themselves are older. The Latin West and the Armenians use unleavened bread, called azymes; the Orthodox use leavened bread, called prosphora; and the Church of the East also uses leavened bread. Traditions thus differ, each defending its own usage.