20221210124000 Octatonic scale

Octatonic scale

An octatonic scale is any 8-note scale. Colloquially it refers to an 8-note scale with alternating semitones and whole tones. In my notes, a reference to an octatonic scale will be referring to the latter of these two definitions.

There are two ways to construct an octatonic scale, where H = half-step (semitone) and W = whole-step (whole tone):

1. H W H W H W H
2. W H W H W H W

This pattern of alternating notes in combination with there being only 12 unique notes (in Western music!) results in there being only 3 unique scales for each construction type:

1. Type 1
   1. C C# Eb E F# G A Bb (C)
   2. C# D E F G Ab Bb B (C#)
   3. D Eb F F# Ab A B C (D)
2. Type 2
   1. C D Eb F F# G# A B (C)
   2. C# D# E F# G A Bb C (C#)
   3. D E F G Ab Bb B C# (D)

To clarify that point, if we were to start a “Type 1” (initial semitone) scale on Eb we’d get:

Eb E F# G A Bb C Db (Eb)

Which is a “rotation” of the scale starting on C.

Furthermore, if we look at the “Type 2” (initial whole tone) scale for C, we can see that it’s actually equivalent to a “Type 1” scale for D.

This means that there are only 3 unique octatonic scales – the other scales are modes of these 3.