Banjo Fdim chord (Open G)F diminished on the 5-string banjo: 3 1 0 3, and 4 more shapes

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

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

Sounds: F A♭ C♭ F  ·  Fdim is F, A♭, C♭ (1 · ♭3 · ♭5)

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

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

Open G tuning: gDGBD, the short 5th string first, then strings 4 to 1.