Root, third and seventh: the guide tones fix the quality, so the fifth is dropped. That is what leaves a string free, so pick a tension to put on top.
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.
Root, third and seventh. The third and seventh, the guide tones, are what make the chord major, minor or dominant, so the fifth can go: it says nothing the symbol needs.
Three notes means a spare string, which is the real point. Put a tension on it.
Fdim7 has 5 of them, on strings 6-4-3-2, strings 5-3-2-1, the bottom four strings and the middle four strings, from open position up to the 12th — all in one inversion, so the bass note never moves. The one to learn first is the F shape at the nut.
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 15 drop 2, 10 drop 3 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.