Tritone (Lat. tritonus)
- William Drabkin
The Interval equal to the sum of three whole tones. In equal temperament it is exactly half an octave and can therefore be perceived either as an augmented 4th or a diminished 5th. Since the beginnings of polyphony in the early Middle Ages theorists and composers have changed their attitudes to the tritone and its use more than to any other interval.
In the medieval system of church modes the tritone was most conspicuous as the interval between the final and the fourth degree of the modes on F, the Lydian and Hypolydian. Conjunct progressions that outline the ascending tritone or the descending diminished 5th F–B are not uncommon in Gregorian chant (although tritone leaps are rare; see Gellnick), and the introduction of B♭ in plainsong notation seems to have been a relatively late development. The first known use of the word ‘tritonus’ occurs in the 9th- or 10th-century organum treatise ...