What we now know as tonality, which is the system of keys and scales, and the harmonic progressions, which had been accepted by audiences since at least the end of the Middle Ages, entered a kind of flux. Keys were no longer stable; dissonances began to resolve onto other dissonances (as in the Prelude to Tristan und Isolde), new harmonies began to insert themselves into the old sequences, and the scale expanded from eight notes to the twelve-note chromatic scale, using notes at random from other keys, and constructing sinuous melodic lines that seemed more adapted to dark and solitary emotions than to the cheerful day-light exuberance of choral song.

The crisis deepened during the first quarter of the 20th century, as a result of two striking innovations. The first was that of Debussy, anticipated by Liszt, who began to use the whole-tone scale (the scale without semitones). This scale, emphasizing each note equally, and being without a dominant, is directionless a...