Bb 7b5 (Bb7b5)
Also written as A# 7b5 (A# and Bb are the same key).
Notes in Bb 7b5
Inversions
Same notes as Bb 7b5, 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 (Bb7b5/D)
2nd inversion
E in the bass (Bb7b5/E)
3rd inversion
G# in the bass (Bb7b5/G#)
Open voicings
A close-position chord like Bb 7b5 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
E on the bottom
Drop 3
D on the bottom
Play A#, D, E and G# together and you get Bb 7b5.
A dominant seventh chord with the fifth lowered a semitone, giving it an off-balance quality that sits somewhere between a plain dominant chord and a whole-tone sound. Jazz players use it as a substitute dominant to slip in an unusual color right before a resolution.