The characteristics of the altered scale
The altered scale is built upon the seventh mode of the melodic minor scale which you saw in the previous article. Hence it's the Locrian flat four mode. This mode is also known as the super-Locrian mode.
In the super-Locrian mode you have all the ambiguity of the use of the Locrian mode we mentioned back when reviewing the natural modes of the major scale (see article 52). Obviously, this ambiguity, this instability of the Locrian flat four mode is mainly produced by the absence of the perfect fifth, or more exactly the alteration of the fifth. So far, nothing changes in relation to the Locrian mode from the major scale, the natural Locrian mode. But the alteration of the fourth degree of the altered scale, the famous "b4", corresponds to the alteration of the last degree, which remains intact in the natural Locrian mode, like the tonic of the mode, obviously. In the altered scale, all degrees apart from the tonic are susceptible of creating tension!
"How dare you speak of tensions in the modal system!" Well, that's why you use the altered scale very occasionally in a strictly modal context, just like the Locrian mode (see article 52).
The use of the altered scale
The altered scale is much more commonly used as a modulation in the tonal system. More specifically, the super-Locrian mode is used with enriched dominant seventh chords (see article 33), notably with a ninth and/or a minor thirteenth like in the example below, based on the traditional II-V-I progression of the tonal system:
In the previous example, the diatonic motion upwards from the G 7 b9 b13 goes through all the notes of the G altered scale: G, Ab, Bb, Cb (B), Db, Eb, F, and back to G.
And, as you can see, the main notes of the seventh chord are also there: the root (G in this case), the third (in this case as the flat fourth, Cb o B for tempered instruments) and the seventh (F in this case).
The use of the altered scale also works with chords enriched with an augmented ninth (#9) or an augmented eleventh (#11).
I'll close this article with a tip for improvisers. This tip stems from the weak interval (a semitone) between the tonic of the altered scale and that of the original melodic minor scale. When you are in a situation where you need to use the altered scale, think of it as a melodic minor scale one semitone above. So, for instance, In the previous example imagine an Ab melodic minor which starts at its seventh degree. The result will be strictly the same, but in most cases this will allow you to visualize the notes much more easily.