The Oxford Dictionary of the Christian Church, s.v. patriarch (ecclesiastical), also calls it "a title dating from the 6th century, for the bishops of the five great sees of Christendom". And Merriam-Webster's Encyclopedia of World Religions says: "Five patriarchates, collectively called the pentarchy, were the first to be recognized by the legislation of the emperor Justinian (reigned 527–565)".
"Since there prevails a custom and ancient tradition to the effect that the bishop of Aelia is to be honored, let him be granted everything consequent upon this honor, saving the dignity proper to the metropolitan" (Canon 7)
