Altered Scale

In jazz, the altered scale or altered dominant scale is a seven-note scale that differs from the Locrian mode in having a lowered fourth scale degree. Starting on C, it contains the notes: C, D♭, E♭, F♭, G♭, A♭ and B♭. (This is the C Locrian mode, C-D♭-E♭-F-G♭-A♭-B♭, with F changed to F♭. For this reason, the altered scale is sometimes called the "super locrian mode.") It is the seventh mode of the ascending melodic minor scale. The scale is sometimes spelled with two thirds rather than a flatted fourth scale degree—e.g. C-D♭-E♭-E♮-G♭-A♭-B♭, with E♮ substituting for F♭. In contrast to the term acoustic scale, the term "altered scale" almost always refers to this particular mode of the melodic minor, rather than the scale itself. In this sense, the term "altered mode" would be more accurate. The altered scale is also known as the Pomeroy scale (Bahha & Rawlins 2005, 33) (after Herb Pomeroy (Miller 1996, 35)), the Ravel scale (after Ravel), and the diminished whole-tone scale (due to its resemblance to the whole-tone scale) (Haerle 1975, 15) as well as the dominant whole-tone scale and Locrian flat four (Service 1993, 28).

The altered scale appears sporadically in the works of Debussy and Ravel (Tymoczko 1997), as well as in the works of recent composers such as Steve Reich (see, in particular, the Desert Music). It plays a fundamental role in jazz, where it is used to accompany altered dominant seventh chords starting on the first scale degree. (That is, the scale C-D♭-E♭-F♭-G♭-A♭-B♭ is used to accompany chords such as C-E-G-B♭-F♯, the dominant seventh sharp eleven chord. See: chord-scale system.

The C super locrian scale consists of the notes C D♭ E♭ F♭ G♭ A♭ B♭ C

One way to obtain the altered scale is by raising the tonic of a major scale by a half step; for example, taking the tonic of the B-major scale, B C♯ D♯ E F♯ G♯ A♯ B

raising the tonic by a half step produces the C altered scale, C C♯ D♯ E F♯ G♯ A♯ C

the notes of which are enharmonic (identical, in the equal temperament system) with the notes of the C altered scale.

Like the other modes of the melodic minor ascending or jazz minor scale, the altered scale shares six of its seven notes with an octatonic (or "diminished") scale, and five of the six notes of a whole-tone scale, and thus is occasionally referred to as the "diminished whole tone scale." (For example, the altered scale C-D♭-E♭-F♭-G♭-A♭-B♭ shares all but its A♭ with the octatonic scale C-D♭-E♭-E-F♯-G-A-B♭; while sharing five of the six notes in the whole-tone scale C-D-E-G♭-A♭-B♭.) This accounts for some of its popularity in both the classical and jazz traditions (Callender 1998, Tymoczko 2004).