Columbia subject headings:
Letter writing--History--Congresses.
Letters--History and criticism--Congresses.
Harvard subject headings:
Communication--Social aspects--History--Congresses.
Letter writing--History--Congresses.
Letter writing--Social aspects--History--Congresses.
Princeton subject headings:
Communications--Social aspects--History--Congresses.
Correspondence--History--Congresses.
Correspondence--Social aspects--History--Congresses.