A 7b9 (A7b9)
1+3+5+b7+b9
Notes in A 7b9
A (root)C#EGBb
Inversions
Same notes as A 7b9, with a different chord tone as the lowest note. Useful for smoother movement between chords instead of jumping around the keyboard.
Root position
A in the bass
1st inversion
C# in the bass
2nd inversion
E in the bass
3rd inversion
G in the bass
4th inversion
Bb in the bass
Open voicings
A close-position chord like A 7b9 can sound cluttered in the middle of the keyboard. Jazz players open it up by dropping one note down an octave, spreading the same notes wider without changing the chord.
Drop 2
G on the bottom
Drop 3
E on the bottom
A 7b9 consists of A, C#, E, G and Bb, played at the same time.
A dominant seventh with a flattened ninth stacked on top, tense and dramatic. It shows up constantly in jazz and in flamenco-adjacent harmony, and it resolves with even more urgency than a plain dominant seventh does.