A diminished seventh chord, also known as a full diminished seventh chord, is composed using a root/1st, minor 3rd, diminished 5th and diminished 7th (or R/1-♭3-♭5-7) intervals played simultaneously. Generally written as C⁰7, C⁰7 or Cmi6(♭5), it is good practice to simply use an uppercase C, a super scripted lowercase ⁰ and the number 7 to represent it in writing (ie: C⁰7).
A diminished seventh chord occurs naturally in a harmonized harmonic minor scale on the seventh scale degree, and although it does not occur naturally in a harmonized major scale, it does appear in a major scales parallel harmonic minor. The double flat () 7th is enharmonically equivalent to a major 6th and is one half-step/semi-tone lower than a minor 7th interval, hence the name diminished 7th.
The tonality of a diminished seventh chord is dissonant and distinctly unresolved, it "drives" towards root resolution as opposed to a major seventh chord which is stable and centered.
Diminished seventh chord profile
|Intervals||root/1st, minor 3rd, diminished 5th, diminished 7th or R/1-♭3-♭5-7|
|Stability||Dissonant, unstable, unresolved|
|Common names (examples in C)||C⁰7, Cdim7, CDIM7, Cmi6(♭5)|
Diminished seventh chord table
|Chord name||Root||Minor third||Diminished fifth||Diminished seventh|
|D♭⁰7||D♭||F♭ (E)||A (G)||C (B♭)|
|E♭⁰7||E♭||G♭||B (A)||D (C)|
|F⁰7||F||A♭||C♭ (B)||E (D)|
|G♭⁰7||G♭||B (A)||D (C)||F (E♭)|
|A♭⁰7||A♭||C♭ (B)||E (D)||G (F)|
|B♭⁰7||B♭||D♭||F♭ (E)||A (G)|
|C Diminished seventh chord voicing #2|