Apostolic See

Of the episcopal sees whose foundation was attributed to the Apostles, five became of particular importance in the early centuries of Christianity:

Alexandria, in present-day Egypt, from Saint Mark the Evangelist, who, although he is not called an apostle in the New Testament, is said to have been one of the Seventy Apostles and to have been commissioned as an apostle when he accompanied Saint Paul and Saint Barnabas in their apostolic journeys.
Antioch, in present-day Turkey, from the Apostle Saint Peter.
Constantinople, now Istanbul in present-day Turkey, from the Apostle Saint Andrew.
Jerusalem, in present-day Israel, or Palestine, from the twelve apostles themselves as the original Church and also a succesor of the Apostle Saint James.
Rome, in present-day Italy, from the Apostles Saint Peter and Saint Paul.

Many other episcopal sees too were founded by Apostles. For instance, the Churches in Thessalonica and Corinth were founded by Saint Paul.

In the present day, the See of Rome is the seat of the Catholic Church and the See of Constantinople is the primary See of the (less centralized) Eastern Orthodox Church. Each of the Eastern Sees is the seat of patriarchs from more than one of the Catholic, Eastern Orthodox and Oriental Orthodox Churches. Three patriarchs now claim to hold this office as Saint Mark's successor: the Pope of the Coptic Orthodox Church of Alexandria, the Greek Orthodox Patriarch of Alexandria, and the Coptic Catholic Patriarch of Alexandria. Two claim the title of Patriarch of Constantinople: the Ecumenical Patriarch of Constantinople and the Armenian Patriarch of Constantinople. For Antioch, see Patriarch of Antioch#Current patriarchs, and for Jerusalem, Patriarch of Jerusalem.

"The Apostolic See" is used in the singular to refer to the see whose bishop is seen as the successor of the Apostle Peter.^[1]

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