A# Augmented 7th (A#7#5)
Also written as Bb Augmented 7th (Bb and A# are the same key).
Notes in A# Augmented 7th
Inversions
Same notes as A# Augmented 7th, 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
2nd inversion
F# in the bass
3rd inversion
G# in the bass
Open voicings
A close-position chord like A# Augmented 7th 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
F# on the bottom
Drop 3
D on the bottom
Press A#, D, F# and G# together on the keyboard and you're playing A# Augmented 7th.
A dominant seventh with the fifth raised a semitone, borrowing the floating quality of an augmented triad while keeping the pull of a dominant chord underneath it. Jazz players use it to add an extra streak of tension right before a resolution.