Banjo Fdim chord (Open D)F diminished on the 5-string banjo: 3 2 2 3, and 3 more shapes

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

1. String 4 (D): 3rd fret, middle finger, F
2. String 3 (F♯): 2nd fret, index finger (barre), A♭
3. String 2 (A): 2nd fret, index finger (barre), C♭
4. String 1 (D): 3rd fret, ring finger, F
5. String 5, the short f♯: open, F♯, 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 f♯ rubs against the F in this shape, a semitone away: some players let it ring, some skip it.

With a capo: on the 3rd fret it’s 0 2 5 3 (frets counted from the capo), an open Ddim shape. Spike the 5th string at the 8th fret to keep it in tune with the capo.

Open D tuning: f♯DF♯AD, the short 5th string first, then strings 4 to 1.