Bb 7b9 (Bb7b9)
Also written as A# 7b9 (A# and Bb are the same key).
Notes in Bb 7b9
Inversions
Same notes as Bb 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
D in the bass (Bb7b9/D)
2nd inversion
E# in the bass (Bb7b9/E#)
3rd inversion
G# in the bass (Bb7b9/G#)
4th inversion
B in the bass (Bb7b9/B)
Open voicings
A close-position chord like Bb 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
The notes A#, D, E#, G# and B make up Bb 7b9.
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.