Columbia subject headings:
Catholic Church. Pope (590-604 : Gregory I). Registrum epistularum--Indexes.
Registrum epistularum--Concordances.
Harvard subject headings:
Gregory I, Pope, approximately 540-604. Registrum epistularum--Concordances.
Princeton subject headings:
Gregory I, Pope, approximately 540-604. Registrum epistularum.
Popes--Correspondence.