Bb Diminished (Bbdim)
Also written as A# Diminished (A# and Bb are the same key).
1+b3+b5
Notes in Bb Diminished
A# (root)C#E
Inversions
Same notes as Bb Diminished, 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 (Bbdim/C#)
2nd inversion
E in the bass (Bbdim/E)
The notes A#, C# and E make up Bb Diminished.
Stack two minor thirds and the fifth shrinks along with the third, producing a triad that never feels at rest. Diminished chords are unstable by design, built to lean somewhere else, usually resolving up by a half step. Played alone it can sound almost comical, but tucked between two other chords it's one of the most useful tension devices in tonal music.