We are used to playing passing tones between the notes of the vii7 chord. Almost all are derived from the harmonic minor scale. Yet there is another tone that is very acceptable to our ear but does not belong to the harmonic minor scale. What is the explanation for this divergence? The answer lies in the acceptable sounds that our ear has become accustomed to hearing. In the dominant chord of any minor key (harmonic minor), we are used to hearing and playing both types of the 9th against the four tones of the dominant 7th chord, consisting of the root, 3rd, 5th, 7th.
The two types of ninths played in the harmonic structure are the flat ninth (b9) and the raised ninth(#9).
George Gershwin used both types of ninths very effectively.
The use of the #9 and b9, in a dominant seventh chord type chord(V7), follows a sometimes unwritten rule. The two types of ninths move back and forth in the melodic structure above the dominant seventh chord. Example: E7b9 E7#9 E7b9 E7#9.
In conventional harmony the b9 resolves by 1/2 step to the 5th of the tonic or the #9 resolves to a tone 1 1/2 steps lower. There are times when the #9 or b9 would resolve by being suspended into the next chord, or might just be held into the next chord, and there becoming one of the basic tones of the next chord.
The E7b9 chord consists of the notes E, G#, B, D, F. The notes of the G#dim7 chord are G#, B, D, F. The two chords are inter related. When the raised #9th is used on the dominant seventh type chord the notes in the E7#9 are E, G#, B, D, F##.(F double sharp)
The melodic movement in these two chord types changes from the Fx to the F. Or you might want to think of this enharmonically the Fx being a G. In that event, the movement would be G to F with the notes of the E7 chord being under both sounds.
