[65] See Riemann, Handbuch der Musikgeschichte, I², p. 144 ff.

[66] See article by Dom Germain in Revue de l’art chrétien, 1888.

[67] Strict ‘imitation’ would be extremely difficult in the tetrachordal system. A subject given in one tetrachord could not be imitated exactly in another, because the tetrachords varied from each other by the position of the half-step within them. Compare, for instance, the modern major and minor modes. The answer given in minor to a subject announced in major is not a strict imitation. If, on the other hand, the answer to a subject in a certain hexachord was given in another hexachord, it would necessarily be a strict imitation, since in all hexachords the half-step came between the third and fourth tones, between mi and fa.

[68] Op. cit., Part II.