Which dioceses could be considered patriarchates (that is, dioceses overseen by patriarchs) was a source of controversy between the eastern and western churches. The bishop of Rome maintained that only a diocese established by an apostle (an apostolic see) could be a patriarchate. However, eastern church leaders recognized more pragmatic concerns such as the size of Christian cities and their political importance. After the Council of Chalcedon, Constantinople was granted patriarchate status because it was the residence of the emperor and senate.
During the reign of Justinian in the sixth century, five patriarchates were officially recognized: Rome, Constantinople, Alexandria, Antioch, and Jerusalem. After the Muslim invasions of the 7th century, only Rome and Constantinople held any real power. And although Rome was considered a patriarchate, its bishop came to be known as the pope; thus, for the most part, whenever reference is made to a patriarch in the Middle Ages, it can usually be inferred as the patriarch of Constantinople.
In the later Middle Ages, patriarchates were established in the autocephalous churches in Serbia, Bulgaria and Russia.
