Seventh chords: one more third, and chords learn to lean

Triads were built by skipping: take a note, skip one, take the next, until three notes stand in a stack. A seventh chord just refuses to stop. Skip once more, add a fourth note, and something changes in kind, not just in size: the plain triad was a block, stable and square, and the seventh chord *leans*. Some lean toward home, some lean back in their chair; either way, four-note chords have opinions, and this page introduces the four you will actually meet.

One more third

Pick a flavor, press play, and flip between Triad and +7th while it loops. The bass and the bottom three notes hold still; the only change is the fourth note arriving on top:

Four personalities from one added note. The major 7th shimmers, the flat 7th on a major triad churns, the flat 7th on a minor triad relaxes, and the half-diminished broods. A seventh chord is nothing more than a triad plus one more skip up the scale, and everything in this article follows from which triad you started with and which seventh landed on it.

Four flavors, free with the key

Just as Roman numerals showed the key handing you seven triads for free, the key hands you seven sevenths, one per degree, qualities included. Build them in C major and the extension's labels read: Imaj7, ii7, iii7, IVmaj7, V7, vi7, viiø7 (that slashed circle is the half-diminished mark). The pattern is worth a minute of staring: the two major-7 chords sit on I and IV, the minor 7ths pile up in the middle, the half-diminished hides on the seventh degree, and exactly one degree in the whole key produces a dominant seventh: the fifth. One chord in the key gets the pulling flavor, and it lives a fifth above home. That is not trivia. That is the plot of the next two articles.

The lean

Why does the dominant seventh pull? Play the two presets back to back:

Plain G already leans: its B is the leading tone from Minor scales, one semitone under home. Adding the F loads a second magnet, because B and F together form a tritone, the restless interval that splits the octave exactly in half, and it discharges by contrary motion: B rises a semitone to C while F falls a semitone to E, both landing inside the C chord. One chord, two notes under tension, one satisfying snap. Nearly every "resolving" gesture in tonal music is this mechanism wearing different clothes.

Then there is the great exception. Blues, and everything downstream of it, plays dominant seventh chords on every degree and resolves none of them: the churn is kept as pure color, the lean repurposed as a way of standing. A blues in C sits on C7 all day and calls it home. Theory says restless; the music says comfortable; the music wins.

In your music

The extension's chord recognizer names all four qualities on sight, Cmaj7, C7, Cm7, Cm7b5, in clips and in the Harmonic Timeline, and the Theory Reference panel lists your key's full seventh-chord shelf with its jazz context. The cheapest experiments in this article: take any triad loop you have and add the diatonic seventh to every chord, and hear it turn into soul on the spot; strip the sevenths from a busy progression and hear it go stadium-wide and anthemic, which is why rock and EDM mostly live on triads; and drop exactly one V7 into the bar before your loop returns home, then feel the room tilt toward the downbeat. Why some orders of these chords feel inevitable while others wander, the pull, mapped, is the next article. The one after names the punctuation marks you have been playing all along.