Columbia subject headings:
Opera--Italy--Bologna.
Operas--17th century--Librettos--Bibliography--Union lists.
Operas--18th century--Librettos--Bibliography--Union lists.
Harvard subject headings:
Opera--Italy--Bologna.
Operas--17th century--Librettos--Bibliography--Union lists.
Operas--18th century--Librettos--Bibliography--Union lists.
Princeton subject headings: