Columbia subject headings:
Church history--6th century--Congresses.
Church history--Middle Ages, 600-1500--Congresses.
Gregory I, Pope, approximately 540-604--Congresses.
Harvard subject headings:
Church history--6th century--Congresses.
Church history--Middle Ages, 600-1500--Congresses.
Gregory I, Pope, approximately 540-604--Congresses.
Princeton subject headings:
Gregory I, Pope, approximately 540-604--Congresses.
Popes--Congresses.