Bb Diminished

Bbdim is a solid intermediate shape once open chords feel steady. The first shape is the easiest voicing: 3 fretted strings and 1 open string. Put your ring finger on the 3rd fret of the G string. Put your index finger on the 1st fret of the C string. Put your middle finger on the 1st fret of the A string. Leave the E string open. Strum all four strings and check each one rings cleanly.

Bbdim is Bb, C#, E. That is a diminished triad (root, minor third, diminished fifth). Play them in any order and you still get Bbdim. Bbdim sits in the key of B major.

Tense and unstable: a passing chord that pulls strongly toward resolution. Chord quality is the interval pattern above the root. That pattern is what you hear as diminished.