Catechism of the Catholic Church §887 — excerpt
Neighboring particular Churches who share the same culture form ecclesiastical provinces or larger groupings called patriarchates or regions. The bishops of these groupings can meet in synods or provincial councils.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: Apostolic Constitutions 34 · LG 23 # 3