Columbia subject headings:
Music--Social aspects--Europe--History--Congresses.
Opera--Europe--History--Congresses.
Outdoor concert facilities--Europe--History--Congresses.
Theaters--Europe--History--Congresses.
Harvard subject headings:
Music--Social aspects--Europe--Congresses.
Opera--Europe--Congresses.
Theater and society--Europe--Congresses.
Theater, Open-air--Europe--History--20th century--Congresses.
Theaters--Europe--History--Congresses.
Princeton subject headings:
Music--Social aspects--Europe--History--Congresses.
Opera--Europe--History--Congresses.
Outdoor concert facilities--Europe--History--Congresses.
Theaters--Europe--History--Congresses.