^Benward & Saker (2003). Music in Theory and Practice, Vol. I, p.155. ISBN978-0-07-294262-0.
^Kostka & Payne (1995). Tonal Harmony, p.84. Third Edition. ISBN0-07-300056-6.
^Optima introductio in contrapunctum, c1300; Coussemaker, Edmond (1876), Scriptores de musica medii aevi, Vol. III, 12; as cited in Drabkin, William. "Consecutive fifths, consecutive octaves." Grove Music Online. Oxford Music Online. Oxford University Press.