Columbia subject headings:
Harvard subject headings:
Archives--Germany--North Rhine-Westphalia--Congresses.
North Rhine-Westphalia (Germany)--Genealogy--Congresses.
North Rhine-Westphalia (Germany)--History--Sources--Congresses.
Princeton subject headings:
Archives--Germany--History--20th century--Congresses.
Archives--Germany--North Rhine-Westphalia--Congresses.