Seventh Chord · Theory

Chord Triad Dominant Seventh Major Seventh Minor Seventh Half-Diminished Seventh Diminished Seventh Harmonic Function Cadence

A seventh chord is a triad with one more third on top: four notes, three thirds.

C seventh chord = C–E–G–B

That added note changes the chord's nature entirely. The difference between triads is brightness; the difference between seventh chords is whether they must move — because a seventh is itself an unstable interval.

The five common seventh chords

Name	Content (from C)	Base triad	Found on
[[concept:dominant-seventh	dominant seventh]]	C–E–G–B♭	major + minor seventh
[[concept:major-seventh	major seventh]]	C–E–G–B	major + major seventh
[[concept:minor-seventh	minor seventh]]	C–E♭–G–B♭	minor + minor seventh
[[concept:half-diminished-seventh	half-diminished]]	C–E♭–G♭–B♭	diminished + minor seventh
[[concept:diminished-seventh	diminished seventh]]	C–E♭–G♭–B𝄫	diminished + diminished seventh

The first four can all be stacked from a major scale by degree; the fifth needs chromatic alteration. That is also why the diminished seventh sounds the most imported of them.

The push supplied by the seventh

A seventh chord's core value is propulsion:

Interval	Position	Effect
minor seventh	root to seventh	a soft instability — needs to resolve downward
major seventh	root to seventh	a sharp instability — a strong pull to close
diminished seventh	root to seventh	maximum tension — one such chord holds two tritones

There is also a special internal feature of the dominant seventh: its third and seventh form a tritone between them (E–B♭). That is exactly why it pushes hardest — one note wants to rise while the other wants to fall, and the opposing motions resolve together into a closing gesture (see cadence).

A seventh chord is not an "upgraded triad"

A common misreading treats seventh chords as triads with extras. In fact:

a triad can stand on its own (a tonic triad does);
a seventh chord is inherently unstable — even a major seventh carries a wish to resolve.

So in traditional harmony a seventh chord is nearly always transitional: it appears, it drives, it resolves. Treating one as a destination only became widespread after the Romantic era.

Diagram: the note that makes the difference

Listen: triad against seventh chord

Hear the C major triad (it can stand), then the dominant seventh C–E–G–B♭ (it must move). The extra note is the whole difference.

Common misconceptions

"A seventh chord is a triad plus a seventh note." It is a triad plus a third, giving four notes and three thirds; adding other notes produces suspended or added-tone chords.
"Seventh chords are more advanced triads." They are inherently unstable. A triad can be a destination; a seventh chord is usually a process.
"Every seventh chord must resolve." In traditional harmony yes; in Impressionism and jazz they can linger, or even serve as the endpoint.
"The diminished seventh and the half-diminished seventh are the same." They differ in the seventh: minor in the half-diminished, diminished (a semitone lower) in the diminished seventh. Two entirely different chords.

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Listen in the library

Scales, Arpeggios, and CadencesC13:22Scale and arpeggio exercises spell out seventh chords one note at a time, making the extra note obviousDetailsSound Ye the Trumpet CallC10:35An organ hymn — dominant sevenths are standard at cadences; hear how they push the music homeDetailsSonata in A Minor, K.175C13:50A sonata in A minor — the resolution of a leading-tone seventh is the clearest instance of the ideaDetails

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

Sources

七和弦 = 在三和弦上再叠一个三度（共四个音、三个三度），属和声学通则
七和弦因含七度而必须解决，其具体解决方向由和弦性质与调性位置决定，为通行和声学表述

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

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