A7b9 lowers the 9th of to , one fret above its root: A, C#, E, G and Bb.
That note can fall one fret to the root of a minor home chord. This is a common altered V in a minor cadence, not its only destination.
Also written A7(b9), A7-9 or A7♭9.
Stack the four notes closely, then drop the second voice from the top an octave. It typically lands on four adjacent strings: the guitar’s workhorse voicing.
That result, 5–R–3–7, is the shape usually meant by drop 2. Named by its bass note, as the inversions are here, it is the 2nd inversion.
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.
Contains A7b9 and adds sharp 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.