Banjo B♭dim chord (Double C)B♭ diminished on the 5-string banjo: 1 3 1 2, and 2 more shapes

Frets, string 4 to string 1: 1 3 1 2 (first position)

1. String 4 (C): 1st fret, index finger (barre), D♭
2. String 3 (G): 3rd fret, ring finger, B♭
3. String 2 (C): 1st fret, index finger (barre), D♭
4. String 1 (D): 2nd fret, middle finger, F♭
5. String 5, the short g: open, G, not in the chord

Sounds: D♭ B♭ D♭ F♭  ·  B♭dim is B♭, D♭, F♭ (1 · ♭3 · ♭5)

Found in: B major (vii°), F♯ mixolydian (iii°), C♯ dorian (vi°), G♯ minor (ii°)

The 5th string: the short g adds the 6th over B♭dim, a colour rather than a clash.

Double C tuning: gCGCD, the short 5th string first, then strings 4 to 1.