Columbia subject headings:
Church history--Middle Ages, 600-1500.
Gregory I, Pope, approximately 540-604--Correspondence.
Popes--Correspondence--History.
Harvard subject headings:
Church history--Middle Ages, 600-1500
Gregory I, Pope, approximately 540-604. Registrum epistularum.
Letter carriers--History--to 1500.
Popes--Correspondence--History.
Princeton subject headings: