Columbia subject headings:
Gregorian chants--Instruction and study--Early works to 1800.
Music theory--History--500-1400.
Harvard subject headings:
Gregorian chants.
Music theory--History--15th century.
Music theory--History--500-1400.
Music theory--Medieval, 400-1500.
Princeton subject headings:
Gregorian chants--Instruction and study--Early works to 1800.
Music theory--History--15th century.