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Four notes · 1 3 5 ♭7 · pure 4:5:6:7

The dominant 7th chord

A dominant 7th chord is a major chord with a minor seventh on top. It sounds strong but unfinished and pulls toward the chord a fifth below; tuned pure, it's the ringing barbershop seventh.

How it's built

Stack a major third and two minor thirds: C, E, G and B♭. The name comes from where it usually sits, on the dominant, the fifth note of the key: in C major that's G7, which is G, B, D and F. Between its third and its seventh, B and F, is a tritone, and that tritone is what pulls.

Singing it in tune

Tuned pure, it's 4:5:6:7, harmonics 4 to 7 (the overtones inside one note) of a note two octaves below the root. The third sits 13.7 cents (a cent is a hundredth of a semitone) below the piano's, the fifth 2.0 above, and the seventh, 7:4, 31.2 below: nearly a third of a semitone. That seventh is far enough from the piano's to sound wrong on its own, until all four notes lock and the chord rings.

The tritone between the third and the seventh tunes to 7:5, 17.5 cents narrower than the piano's. Barbershop singers build their sound on this chord, and many talk of a fifth voice: when it locks, the reinforced overtones can sound like a note no one is singing.

Where you hear it

Its classic move is V7 to I, G7 to C. The third, B, rises to C, and the seventh, F, falls to E, so the tritone closes in. In a jazz ii-V-I it's the V: Dm7, G7, Cmaj7.

Barbershop arrangements are full of it, often in chains round the circle of fifths: E7, A7, D7, G7, then C. The blues puts it on every chord, C7, F7 and G7, with no need to resolve.

More chord types

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Stackapella recording the alto part of Amazing Grace: each note is a bar with its name, and the one being sung says A little low, go up. Stackapella recording the alto part of Amazing Grace: each note is a bar with its name, and the one being sung says A little low, go up. Stackapella recording the alto part of Amazing Grace: each note is a bar with its name, and the one being sung says A little low, go up. Stackapella recording the alto part of Amazing Grace: each note is a bar with its name, and the one being sung says A little low, go up.
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Questions

Why is it called a dominant 7th?

Because it's the seventh chord built on the dominant, the fifth note of a major or harmonic minor scale. In C that's G7, and the same shape on any root keeps the name.

What is the difference between a dominant 7th and a major 7th chord?

The seventh. A dominant 7th has the minor seventh, 10 semitones up (B♭ over C), and a major 7th has the major seventh, 11 up (B). That one semitone changes a chord that pulls into one that rests.

What is a barbershop seventh?

A dominant 7th tuned pure, 4:5:6:7, with its seventh 31.2 cents below the piano's. The low seventh lines its overtones up with the other notes', so the chord stops beating and rings.

Why does a dominant 7th want to resolve?

Because of the tritone between its third and seventh. In G7 that's B and F, and the strongest way to settle it is B up to C and F down to E, which lands on the C major chord.

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