As its name suggests, the octatonic scale is an eight note scale constructed from recurring semitones and tones.
Octatonic scales are symmetrical: the intervallic structure of the first four notes is the same as the structure of the last four notes – semitone, tone, semitone.
The octatonic scale divides the octave equally: into minor thirds (written as an enharmonic equivalent, the D sharp in the following example, is a minor third above the initial C note. Throughout this article, enharmonic equivalents will be used to clarify examples and for ease of reading).
and, into tritones.
There are only three octatonic scales, beginning on the notes C, C sharp and D.
As with these examples, the octatonic scale may begin with a semitone – between the first and second notes – or a tone. When it begins with a tone, the resulting tone -semitone octatonic scale is simply a mode of one of the three scales shown above. Because of this fact, this article will only refer to the three octatonic scales beginning with semitones.
When harmonised, all three octatonic scales have diminished seventh chords on the first two notes of the scale, leading some to call them diminished scales (this term is particularly favoured by Jazz musicians).
Excluding extended chords (such as ninths, elevenths and thirteenths) and altered forms of seventh chords, a harmonised octatonic scale produces the following thirty chords.
As can be seen, the array of diminished, half diminished, and dominant seventh chords has allowed many composers to use the octatonic scale for a range of functions. For example, considering only the octatonic scale’s two diminished seventh chords and their unique ability to modulate to four different keys, the octatonic scale can connect eight major keys and their parallel minor keys.
An interesting fact about the eight modulatory keys of the diminished seventh chords in the C octatonic scale, is their root notes form the octatonic scale beginning on C sharp
and the root notes of this scale’s eight keys forms the octatonic scale beginning on D.
The root notes of the D octatonic scale’s diminished seventh chords forms the C octatonic scale. In this way, the octatonic scales allow easy modulation through all twelve major and minor keys, without even considering the modulatory potential of the half diminished seventh and dominant seventh chords also contained in these scales.
The octatonic scale’s four dominant seventh chords would resolve to the following keys; the root notes of which form a diminished seventh chord.
The relationship of these dominant seventh chords have been used by composers to divide the octave into dominant sevenths related by thirds rather than the more common relationship of the fourth or fifth.
The keys of resolution of the octatonic scale’s four half diminished seventh chords also create a diminished seventh chord.
Melodically, the octatonic scales contain scale degrees 6 – 7 – 1 of four major scales, and their parallel melodic minor scales.
In each octatonic scale, the root notes of these four scale fragments create the same diminished seventh chord produced by the resolutions of the scale’s half diminished seventh chords.
The intervallic relationship between the octatonic scale’s 6 – 7 – 1 scale degrees (tone – semitone) can also be heard as scale degrees 2 – 3 – 4 in a major key.
While scale degrees 7 – 1 – 2 (semitone – tone) can also be heard as 3 – 4 – 5 in a major key.
This concept can also be extended to the three minor scale types: