Bb Diminished 7th (Bbdim7)
Also written as A# Diminished 7th (A# and Bb are the same key).
Notes in Bb Diminished 7th
Inversions
Same notes as Bb Diminished 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
C# in the bass (Bbdim7/C#)
2nd inversion
E in the bass (Bbdim7/E)
3rd inversion
G in the bass (Bbdim7/G)
Open voicings
A close-position chord like Bb Diminished 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
E on the bottom
Drop 3
C# on the bottom
The notes A#, C#, E and G make up Bb Diminished 7th.
Stack three minor thirds in a row and the seventh shrinks all the way to a double-flatted seventh, which lands on the same key as a plain sixth. The chord is perfectly symmetrical, so the same four keys can be renamed and reused as a diminished seventh built on any of its four notes. It's the classic villain-sting and passing chord of common-practice harmony.