This study reviews how the term "Apostolic See" was used in the documents of the Second Vatican Council and in post-Conciliar documents including the new "Codex Canonum Ecclesiarum Orientalium" which opts, with but one exception, for the use of "Apostolic See" when naming the Roman Apostolic See. The Second Vatican Council's consistent use of "Sedes Apostolica Romana" in "Unitatis redintegratio" implicitly recognizes other Apostolic Sees besides the Apostolic See of Rome, whereas the new Eastern canonical legislation uses "Sedes Apostolica" without qualification 149 times. While this unqualified use of the term "Apostolic See" for Rome persists, the study discloses that there was a growing awareness on the part of Conciliar Fathers and the Popes themselves following the Council to qualify the exclusive formula. To help explain the origin and spread of the exclusive usage of "Apostolic See", the thesis adopts an interdisciplinary approach using insights from the sciences of ecclesiology, linguistics and sociology. Applying the sociological concept of reference groups, the study argues that the Eastern churches initially conformed to the Roman usage as the model to imitate.
However, a more recent sampling of the use of "Apostolic See" by the Maronite, Melkite and Coptic Catholic Churches reveals that these churches have not invariably adopted the Western practice of naming Rome the "Apostolic See". The work concludes with the wish that the new Eastern code's adoption of the Roman terminology does not obscure its genuinely Eastern and ecumenical character.