A7b9 is A, C#, E, G and Bb — with the ninth lowered a semitone.
It sounds dark and urgent. The flat ninth comes from the harmonic minor, so this is the V of a minor key rather than a major one.
Altered dominant — it points at a minor chord.
Also written A7(b9), A7-9 or A7♭9.
A7b9 is the dominant a minor key builds from its own notes. The flat nine is the key’s flat sixth, a half step above the fifth of Dm, and it falls there.
Listen for: The flat nine falls a half step to the fifth of the minor chord. On the plain seventh chord that string holds the root.
Every note of A7b9 is in A half-whole diminished.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where A7b9 sits in it. Every scale.
Change one note of A7b9 and you have (drop the flat ninth), (the flat ninth becomes the ninth), (the flat ninth becomes the sharp ninth) or (drop the flat ninth).
A7b9 is an altered chord — a dominant seventh with its fifth or ninth moved. Altered dominant chords on every root.
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.