The third voice from the top lowered an octave. The dropped voice becomes the bass, which is why a string is skipped.
Fdim7 is F, Ab, Cb and D — Fdim with a diminished seventh stacked on top.
It sounds suspenseful and rootless. Three stacked minor thirds divide the octave evenly, so the same four notes spell four dim7 chords and the shape repeats every three frets.

| F | Ab | Cb | D |
|---|---|---|---|
| 1 | ♭3 | ♭5 | ♭♭7 |
Leading-tone or passing chord — it can resolve almost anywhere.
Also written F°7, Fo7 or F diminished 7. The same chord either way; which spelling you meet depends on who wrote the chart.
The same operation one voice further down: the third voice from the top falls an octave and becomes the bass.
That leaves a bigger hole under the top three notes, so a string gets skipped: the muted string in the middle is the shape, not a mistake.
Fdim7 has 10 of them, on strings 5-3-2-1 and strings 6-4-3-2, from open position up to the 13th — every inversion the chord has: F in the bass, Ab in the bass, Cb in the bass and Ebb in the bass. The one to learn first is the Cb shape at the nut.
5 of them sit on strings 5-3-2-1, so you can run the inversions up one set without changing strings — which is the whole reason to learn them as a family rather than as separate grips.
Start with xx3434 — no barre and low, at the 3rd fret. 1 of the 8 shapes here sits in open position. 4 of the 8 need a barre. Together they cover the neck from the 1st fret to the 15th.
| Frets | Where | Bass | Top | Hand |
|---|---|---|---|---|
| 1x0101 | open position | F, root position | F | barred |
| 123131 | 1st fret | F, root position | F | barred |
| xx3434 | 3rd fret | F, root position | Ab | all fretted |
| xx6767 | 6th fret | Ab, 1st inversion | Cb | all fretted |
| x89797 | 7th fret | F, root position | Cb | all fretted |
| x-x-9-10-9-10 | 9th fret | Cb, 2nd inversion | D | all fretted |
| 13-x-12-13-12-13 | 12th fret | F, root position | F | barred |
| 13-14-15-13-15-13 | 13th fret | F, root position | F | barred |
The same notes rebuilt from scratch give 5 shell, 15 drop 2 and 6 open string voicings, each on its own page.
Tensions. A shell of Fdim7, root, third and seventh, leaves a string free, and the tones that belong over it are the 9 (G), the 11 (Bb), the b13 (Db) and the maj7 (E).
A diminished seventh divides the octave evenly, so Fdim7 has no single root and no single destination: it can lead to Bb or to Gb, and it is the same four notes either way.
Every note of Fdim7 is in F diminished and F half-whole diminished.
Change one note of Fdim7 and you have Fdim (drop the sixth), Fm6 (the flat fifth becomes a fifth), Fm7b5 (the sixth becomes a flat seventh) or FmMaj7b5 (the sixth becomes a major seventh).
Fdim7 is a diminished chord — root, minor third, flat fifth. Diminished and half-diminished chords on every root.
On Fdim7’s sound and use this page follows Stefan Kostka, Dorothy Payne & Byron Almén, Tonal Harmony (McGraw-Hill, 8th ed., 2018). The shapes and the derived voicings come from different places, and both are explained in where the chords and the theory come from.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.