Columbia subject headings:
Communication and culture--History.
Communication--Social aspects--History.
Harvard subject headings:
Civilization, Medieval--Congresses
Communication and culture--History--Middle Ages, 600-1500--Congresses.
Communication--History--Congresses.
Communication--Social aspects--History--Middle Ages, 600-1500--Congresses.
Diplomatic couriers--History--Middle Ages, 600-1500--Congresses.
Princeton subject headings:
Civilization, Medieval--Congresses.
Communication--Europe--History--Congresses.