Bdim7 guitar chord
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.
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About Bdim7 — shell voicings
Bdim7 is B, D, F and Ab — Bdim 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.
Bdim7 notes and formula
| B | D | F | Ab |
|---|---|---|---|
| 1 | ♭3 | ♭5 | ♭♭7 |
Leading-tone or passing chord — it can resolve almost anywhere.
Also written B°7, Bo7 or B diminished 7. The same chord either way; which spelling you meet depends on who wrote the chart.
How to play Bdim7
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.
Bdim7 has 7 of them, on strings 5-3-2-1, strings 6-4-3-2, the middle four strings and the bottom four strings, from open position up to the 13th — all in one inversion, so the bass note never moves. The one to learn first is the B shape at the nut.
Start with xx3434 — no barre and low, at the 3rd fret. None of the shapes for Bdim7 uses an open string, so every one of them is movable; the lowest sits at the 1st fret. 2 of the 7 need a barre. Together they cover the neck from the 1st fret to the 15th.
| Frets | Where | Bass | Top | Hand |
|---|---|---|---|---|
| x23131 | 1st fret | B, root position | F | all fretted |
| xx3434 | 3rd fret | F, 2nd inversion | Ab | all fretted |
| 7x6767 | 6th fret | B, root position | B | barred |
| 789797 | 7th fret | B, root position | B | barred |
| x-x-9-10-9-10 | 9th fret | B, root position | D | all fretted |
| x-x-12-13-12-13 | 12th fret | D, 1st inversion | F | all fretted |
| x-14-15-13-15-13 | 13th fret | B, root position | F | all fretted |
The same notes rebuilt from scratch give 15 drop 2, 10 drop 3 and 5 open string voicings, each on its own page.
Tensions. A shell of Bdim7, root, third and seventh, leaves a string free, and the tones that belong over it are the 9 (C#), the 11 (E), the b13 (G) and the maj7 (A#).
How to use Bdim7
A diminished seventh divides the octave evenly, so Bdim7 has no single root and no single destination: it can lead to E or to C, and it is the same four notes either way.
What to play over Bdim7
Every note of Bdim7 is in B diminished and B half-whole diminished.
Chords near Bdim7
Change one note of Bdim7 and you have Bdim (drop the sixth), Bm6 (the flat fifth becomes a fifth), Bm7b5 (the sixth becomes a flat seventh) or BmMaj7b5 (the sixth becomes a major seventh).
Bdim7 is a diminished chord — root, minor third, flat fifth. Diminished and half-diminished chords on every root.
On Bdim7’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.