What Is a Diminished 7th Chord?
A diminished 7th chord is a diminished triad with a diminished 7th interval added above the root. Every interval is exactly 3 semitones (a minor third) — making this the most symmetrical chord in Western harmony.
The diminished 7th is the chord of maximum harmonic tension. Unlike the half-diminished 7th (which has a minor 7th), the fully diminished 7th has a diminished 7th — an interval that sounds like a major 6th but is written as a 7th. This extra compression creates an almost unbearable tension that demands release.
How to Build a Diminished 7th Chord
Formula
Root + Minor 3rd + Diminished 5th + Diminished 7th
Semitones: 0 — 3 — 6 — 9
Example: C Diminished 7th
- C (root)
- E♭ (3 semitones = minor third)
- G♭ (6 semitones = diminished fifth)
- B♭♭ (= A) (9 semitones = diminished seventh)
Enharmonic note: B♭♭ is the correct spelling (diminished 7th above C) but it sounds identical to A. In practice, you'll often see it written as A.
The Symmetry of Diminished 7th Chords
Because each interval is exactly 3 semitones, a diminished 7th chord divides the octave into four equal parts. This means there are only three unique diminished 7th chords — every other "different" diminished 7th is just an enharmonic respelling:
Group 1: C°7 = E♭°7 = F#°7 = A°7 (same 4 notes, different roots)
Group 2: C#°7 = E°7 = G°7 = B♭°7
Group 3: D°7 = F°7 = A♭°7 = B°7
All 12 Diminished 7th Chords
| Chord | Root | Minor 3rd | Dim. 5th | Dim. 7th |
|---|---|---|---|---|
| C°7 | C | E♭ | G♭ | B♭♭(A) |
| G°7 | G | B♭ | D♭ | F♭(E) |
| D°7 | D | F | A♭ | C♭(B) |
| A°7 | A | C | E♭ | G♭(F#) |
| E°7 | E | G | B♭ | D♭(C#) |
| B°7 | B | D | F | A♭(G#) |
| F#°7 | F# | A | C | E♭(D#) |
| F°7 | F | A♭ | C♭(B) | E♭♭(D) |
| B♭°7 | B♭ | D♭ | F♭(E) | A♭♭(G) |
| E♭°7 | E♭ | G♭ | B♭♭(A) | D♭♭(C) |
| A♭°7 | A♭ | C♭(B) | E♭♭(D) | G♭♭(F) |
| D♭°7 | D♭ | F♭(E) | A♭♭(G) | C♭♭(B♭) |
The Sound of a Diminished 7th Chord
Extremely tense, eerie, ambiguous, dramatic, unstable.
The diminished 7th is the chord of maximum dissonance. Its perfect symmetry means it has no clear tonal centre — it could resolve to any of four different keys. Composers exploit this ambiguity for dramatic modulations and for moments of extreme tension before resolution.
How to Use Diminished 7th Chords
1. As a leading-tone chord: In minor keys, the vii°7 resolves powerfully to the i chord. B°7 → Cm is an extremely strong cadence.
2. For modulation: Because a diminished 7th can resolve to four different keys, use it as a pivot chord to modulate to a distant key smoothly.
3. In jazz passing chords: Diminished 7th chords work as chromatic passing chords between adjacent diatonic chords.
4. For dramatic effect: Film composers and classical composers use diminished 7ths at moments of maximum tension, horror, or surprise.
Free: Chord Ear Training Cheat Sheet
Train your ear to recognise the eerie, symmetrical sound of fully diminished 7th chords.