Columbia subject headings:
Music--Germany--North Rhine-Westphalia--Congresses.
Harvard subject headings:
Music--Germany--North Rhine-Westphalia--Congresses.
Princeton subject headings:
Music--20th century--History and criticism--Congresses.
Music--Germany--North Rhine-Westphalia--History and criticism--Congresses.
Opera--Germany--North Rhine-Westphalia--Congresses.