Diminished Seventh

Seventh Chord Half-Diminished Seventh Diminished Triad Augmented Triad Augmented and Diminished Intervals Cadence

A diminished seventh is a diminished triad with one more minor third on top — but the result is not merely "one more note":

C diminished seventh = C–E♭–G♭–B𝄫 (B𝄫 means "B flattened twice", sounding as A)

The four notes are equally spaced (each step a minor third, 3 semitones). That equidistance has three unusual consequences.

Consequence one: only three of them

The structure repeats every three semitones, so scales from C, C♯ and D cover everything — there are only three, the rest being enharmonic repeats. This is the same phenomenon as the augmented triad (only four forms): an equidistant structure must repeat itself.

Consequence two: two tritones

Among the four notes you can find two separate pairs six semitones apart:

Pair	Interval
C ↔ G♭ (root and fifth)	diminished fifth
E♭ ↔ B𝄫 (third and seventh)	diminished seventh (enharmonically a major sixth)

So the diminished seventh is the least stable chord at the level of intervals. Once it appears, resolution is almost compulsory.

Consequence three: no single root

With four equal steps, any note can be heard as the root. Two practical results follow:

it sounds unmistakably unstable, which is why it is a fixture of horror and suspense scoring and of silent

film accompaniment;

it is one of the handiest pivots for modulation — the same chord can belong to four different keys, and

reinterpreting it turns the music elsewhere (see modulation).

Diagram: three equidistant structures compared

Listen: the diminished seventh

Notice how it hangs in mid-air — more tense than a diminished triad, because it contains two tritones.

Common misconceptions

"Diminished and half-diminished sevenths are the same." Their sevenths differ: minor in the half-diminished, a semitone lower in the diminished seventh. The heard difference is plain — see half-diminished seventh.
"There are twelve diminished sevenths." Only three; the rest are enharmonic repeats.
"B𝄫 must be a typo." It is a legal double flat. Writing A would break the every-other-letter stacking.
"It is only good for horror." That is one use. It is equally a modulation pivot and one of the Romantic era's commonest propulsive devices.

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Listen in the library

Scales, Arpeggios, and CadencesC13:22Scale exercises can sound a diminished-seventh arpeggio, letting you hear its four equal sidesDetailsSonata in A Minor, K.175C13:50A sonata in A minor — the diminished seventh on degree 7 resolving to the tonic, its most typical useDetailsDanse Macabre, S.555C110:20Liszt's transcription of Danse macabre — chromatic harmony often uses diminished sevenths for unease and as pivotsDetails

All tracks from midicn-lib — inline badge is the license tier (C1 commercial / C2 non-commercial / C3 study only).

Sources

减七和弦 = 三个减三度叠置（共四个音、根音到七音为减七度），属和声学通则
因结构等距对称而只有三种不同形态、且含两个三全音，为通行和声学表述

Used for fact-checking only; all prose is original.

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