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. (August 2015)
The Orthodox Church is a communion comprising the fifteen separate autocephalous hierarchical churches that recognize each other as "canonical" Orthodox Christian churches. Each constituent church is self-governing; its highest-ranking bishop (a patriarch, a metropolitan or an archbishop) reports to no higher earthly authority. Each regional church is composed of constituent eparchies (or dioceses) ruled by bishops. Some autocephalous churches have given an eparchy or group of eparchies varying degrees of autonomy (self-government). Such autonomous churches maintain varying levels of dependence on their mother church, usually defined in a Tomos or other document of autonomy. In many cases, autonomous churches are almost completely self-governing, with the mother church retaining only the right to appoint the highest-ranking bishop (an archbishop or metropolitan) of the autonomous church.
Normal governance is enacted through a synod of bishops within each church. In case of issues that go beyond the scope of a single church, multiple self-governing churches send representatives to a wider synod, sometimes wide enough to be called an Orthodox "ecumenical council". Such councils are deemed to have authority superior to that of any autocephalous church or its ranking bishop.
The Orthodox Church is decentralised, having no central authority, earthly head or a single Bishop in a leadership role. Thus, the Orthodox Church uses a synodical system canonically, which is significantly different from the hierarchically organised Catholic Church that follows the doctrine of papal supremacy. References to the Ecumenical Patriarch of Constantinople as a leader are an erroneous interpretation of his title ("first among equals"). His title is of honor rather than authority and in fact the Ecumenical Patriarch has no real authority over Churches other than the Constantinopolitan. His unique role often sees the Ecumenical Patriarch referred to as the "spiritual leader" of the Orthodox Church in some sources, though this is not an official title of the patriarch nor is it usually used in scholarly sources on the patriarchate.
The autocephalous churches are in full communion with each other, so any priest of any of those churches may lawfully minister to any member of any of them, and no member of any is excluded from any form of worship in any of the others, including reception of the Eucharist.
In the early Middle Ages, the One Holy Catholic and Apostolic Church was ruled by five patriarchs: the bishops of Rome, Constantinople, Alexandria, Antioch, and Jerusalem; these were collectively referred to as the Pentarchy. Each patriarch had jurisdiction over bishops in a specified geographic region. This continued until 927, when the autonomous Bulgarian Archbishopric became the first newly promoted patriarchate to join the original five.
The patriarch of Rome was "first in place of honor" among the five patriarchs. Disagreement about the limits of his authority was one of the causes of the Great Schism, conventionally dated to the year 1054, which split the church into the Catholic Church in the West, headed by the Bishop of Rome, and the Orthodox Church, led by the four eastern patriarchs. After the schism this honorary primacy shifted to the Patriarch of Constantinople, who had previously been accorded the second-place rank at the First Council of Constantinople.