Banjo Cdim chord (Double C)C diminished on the 5-string banjo: 0 5 3 4, and 3 more shapes

Frets, string 4 to string 1: 0 5 3 4 (open position)

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

Sounds: C C E♭ G♭  ·  Cdim is C, E♭, G♭ (1 · ♭3 · ♭5)

Found in: D♭ major (vii°), A♭ mixolydian (iii°), E♭ dorian (vi°), B♭ minor (ii°)

The 5th string: the short g rubs against the G♭ in this shape, a semitone away: some players let it ring, some skip it.

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