Columbia subject headings:
Harvard subject headings:
Catholic Church--Liturgy--History--Middle Ages, 600-1500.
Codex Angelicus 123.
Graduals (Liturgical books)
Gregorian chants--History and criticism.
Tropers.
Princeton subject headings:
Codex Angelicus 123.
Gregorian chants--History and criticism.
Music--Manuscripts--Italy--Rome.
Neumes--Italy--Bologna.