Bb 13b9 (Bb13b9)
Also written as A# 13b9 (A# and Bb are the same key).
Notes in Bb 13b9
Inversions
Same notes as Bb 13b9, 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 (Bb13b9/D)
2nd inversion
E# in the bass (Bb13b9/E#)
3rd inversion
G# in the bass (Bb13b9/G#)
4th inversion
B in the bass (Bb13b9/B)
5th inversion
G in the bass (Bb13b9/G)
Open voicings
A close-position chord like Bb 13b9 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
A# on the bottom
Drop 3
G# on the bottom
Play A#, D, E#, G#, B and G together and you get Bb 13b9.
A dominant thirteenth with a flattened ninth. The b9 adds real bite and urgency, which makes the eventual resolution feel stronger. It is at home in jazz turnarounds and in flamenco tinged progressions where the dominant needs extra drama.