Columbia subject headings:
Operas--Excerpts--Vocal scores with piano.
Operas--Librettos.
Harvard subject headings:
Excerpts
Librettos
Operas
Operas--Excerpts--Vocal scores with piano
Operas--Librettos
Perceval (Legendary character)--Drama--Librettos.
Tannhäuser--Drama--Librettos.
Princeton subject headings:
Excerpts.
Librettos.
Operas--Excerpts--Vocal scores with piano.
Operas--Librettos.