A7#9 holds the major 3rd and in the same dominant chord: A, C#, E, G and B#. The two notes sound a semitone apart.
The major 3rd stays essential. In blues, 7♯9 can stand on the tonic itself as well as lead to another chord.
Also written A7(#9), A7+9 or A7♯9.
A7#9 in 3rd inversion puts G in the bass instead of A, the 7th of the chord. The notes are the same; what changes is what the bass line does, which is most of why you would reach for it — it lets the lowest voice move by step where the root would have jumped.
One of A7#9’s 10 shapes does this:xx5658
A7#9 can replace a plain seventh chord wherever a bluesy sound is wanted, even as the first chord of a tune, where other altered dominants don’t belong. It sounds like home, with no urge to fall a fifth.
Listen for: The sharp nine, a minor third sounding over the major third, holds through the IV7 as its flat seventh.
Like the other altered dominants, A7#9 works in front of a chord a fourth higher, such as Dmaj7, a chord a half step lower, or a minor chord on its own root.
Contains A7#9 and adds flat ninth, sharp eleventh and thirteenth.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.