Columbia subject headings:
Gregorian chants--Congresses.
Music printing--Congresses.
Service books (Music)--Catholic Church--Congresses.
Harvard subject headings:
Church music--Catholic Church
Gregorian chants--Congresses.
Gregorian chants--History and criticism--Congresses.
Music printing--Congresses.
Sacred vocal music--Italy
Service books (Music)--Catholic Church--Congresses.
Princeton subject headings:
Gregorian chants--Congresses.
Music printing--Congresses.
Service books (Music)--Catholic Church--Congresses.