The Dominant chord is the real core of the tonal system, with its inner tension to resolve to the tonic, due to the presence of the tritone interval in its structure.
Improvisers and composers have increasingly stressed this tension by adding extensions and alterations to the basic 1-3-5-b7 structure. One of the many possibilities is represented by the alteration of the 9th.
In functional “European” harmony, ninth chords first appeared on the V degree of the scale, that is the Dominant. Just like it was the Dominant triad the first being added with the 7th. In both cases it served the purpose of making the resolution to the Tonic even more powerful.
Let’s remember that, every time we are talking about harmony, we are also dealing with voice leading and so with melodic thinking. Alterations should follow a “linear logic” that leads to the next chord.
Let’s first consider the harmonic minor scale, where we find a 7(b9) chord on the V degree. Starting from A:
The same chord comes from the harmonization of the diminished scale:
In this case, we are dealing with a symmetrical scale, meaning that everything is repeated four times, a minor third apart.
Another scale often used over altered Dominant chords, is the superlocrian, the seventh mode of the melodic minor scale.
Here’s how the 7(b9) chord can be played on the guitar:
Back to the diminished scale, the four Dominant chords also have the sharp ninth.
On the guitar: