Bb7#9 holds the major 3rd and in the same dominant chord: Bb, D, F, Ab and C#. 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 Bb7(#9), Bb7+9 or Bb7♯9.
Bb7#9 in 3rd inversion puts Ab in the bass instead of Bb, 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 Bb7#9’s 10 shapes does this:xx6769
Bb7#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, Bb7#9 works in front of a chord a fourth higher, such as Ebmaj7, a chord a half step lower, or a minor chord on its own root.
Contains Bb7#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.