Columbia subject headings:
Harvard subject headings:
Church history--Primitive and early church, ca. 30-600
Church history--Primitive and early church, ca. 30-600.
Gregory I, Pope, approximately 540-604. Registrum epistularum.
Popes--Correspondence
Princeton subject headings:
Gregory I, Pope, approximately 540-604. Registrum epistularum.
Popes--Correspondence.