Columbia subject headings:
Opera--19th century.
Opera--20th century.
Harvard subject headings:
Opera--19th century
Opera--20th century
Opera--Germany
Opera--Italy
Realism in music.
Verismo (Opera)--Germany.
Princeton subject headings:
Opera--Austria--19th century.
Opera--Austria--20th century.
Opera--Germany--19th century.
Opera--Germany--20th century.
Realism in music.