Four notes · 1 ♭3 ♭5 𝄫7 · pure 15:18:21:25
The diminished 7th chord
A diminished 7th chord is three minor thirds stacked, splitting the octave into four equal parts on a piano. It sounds tense and suspenseful, and it can lead almost anywhere.
How it's built
On C it's C, E♭, G♭ and B𝄫. The double flat is a seventh lowered twice, and it sounds as A; spelled as a stack of thirds it needs the double flat, though many chord charts simply write A.
On a piano every gap in it is a minor third, including the one from the top note up to the next C. So C, E♭, G♭ and A diminished 7th use the same four keys, and a piano has only three different diminished 7th chords.
Singing it in tune
Pure tuning breaks the symmetry, because it's measured from the root. Switched to Pure, this page tunes it 15:18:21:25: the minor third 15.6 cents (a cent is a hundredth of a semitone) above the piano's, the diminished fifth, 7:5, 17.5 below, and the diminished seventh, 5:3, 15.6 below. Its four thirds come out unequal, from 266.9 cents between the third and the fifth to 315.6 at the bottom and the top.
So there's no single pure version that suits every root. Move the root with − and + and the same four piano keys tune differently, depending on which one is treated as the root.
Where you hear it
In a minor key it's the chord on the seventh note: B, D, F and A♭ in C minor, pulling to C minor. Any of its notes can act as the leading note (the note just below the home note) of a different key, which made it a favourite for changing key in Romantic music, and for suspense in melodrama and silent film. In barbershop and close harmony it often appears as a passing chord.
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Questions
Why does a diminished 7th chord have a double flat?
Because its seventh is diminished: a minor seventh lowered one more semitone. On C, the major seventh is B, the minor seventh B♭, and the diminished seventh B𝄫, which sounds as A.
Why are there only three diminished 7th chords?
Because on a piano it splits the octave into four equal minor thirds, so each chord shares its notes with three others. Starting on C, C♯ and D covers them all; on D♯ you're back to C's notes.
What is the difference between diminished 7th and half-diminished?
The seventh. A diminished 7th has a diminished seventh, 9 semitones up (B𝄫 over C), and a half-diminished has a minor seventh, 10 up (B♭). The diminished 7th is tenser and more symmetrical.
Is dim the same as dim7?
No: Cdim is the three-note diminished chord, C, E♭ and G♭, and Cdim7 adds the diminished seventh. Some chord charts write dim when they mean dim7, so check the arrangement.
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