Columbia subject headings:
Music--Economic aspects--Europe.
Music--Europe--20th century--History and criticism.
Music--Europe--20th century--Statistics.
Harvard subject headings:
Music--Economic aspects--Europe.
Music--Europe--20th century--History and criticism
Music--Europe--20th century--Statistics.
Opera--Europe--20th century.
Princeton subject headings:
Music--Economic aspects--Europe.
Music--Europe--20th century--History and criticism.
Music--Europe--20th century--Statistics.