Apel, W. (1958). Gregorian chant. Indiana University Press.
Lua i Stíl Chicago (17ú heag.)Apel, Willi. Gregorian Chant. Bloomington: Indiana University Press, 1958.
Lua MLA (9ú heag.)Apel, Willi. Gregorian Chant. Indiana University Press, 1958.
Rabhadh: Seans nach mbeach na luanna seo go hiomlán cruinn i ngach uile chás.