A9 is A, C#, E, G and B — with the ninth stacked on top.
It sounds bright and full, still clearly pulling. The ninth adds height without softening the tritone underneath it, which is why funk and soul comp on this rather than on a plain 7.
Dominant — the same pull as a 7, with more colour.
Play it instead of a m7♭5 a major third above this root, something else has the bass, since that chord is this one without its root.
Also written Adom9 or A dominant 9.
A9 goes wherever a dominant seventh goes; the ninth is only an added colour. When the chord resolves, the ninth does what the seventh does and falls a step.
Listen for: The ninth falls a whole step to the fifth of the I. On the plain seventh chord that string plays the root, which just holds.
In which keys. Every note of A9 belongs to D major and B minor.
Every note of A9 is in A mixolydian and A lydian dominant.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where A9 sits in it. Every scale.
Change one note of A9 and you have (the flat seventh becomes the major seventh), (the major third becomes the minor third), (drop the flat seventh), (the flat seventh becomes the sixth), (drop the ninth) or (the fifth becomes the flat fifth).
A9 is a dominant chord — root, major third, fifth, minor seventh. Dominant seventh 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.