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Three notes · 1 4 5 · pure 6:8:9

The sus4 chord

A sus4 chord swaps the third for the fourth: a root, the note a perfect fourth above it, and the fifth. It sounds open and expectant, and it often settles by letting the fourth fall to the third.

How it's built

Take a major chord and replace its third with the fourth: C, F and G. The fourth clashes with the root below it, and that clash is what asks it to move. With no third it's neither major nor minor until the fourth comes down.

Singing it in tune

Tuned pure, it's 6:8:9. The fourth, 4:3, sits 2.0 cents (a cent is a hundredth of a semitone) below the piano's and the fifth 2.0 cents above, so the whole step between them, 9:8, is 3.9 cents wider than the piano's.

When it resolves, the singer on the fourth steps down to the major third, and in pure tuning that third sits 13.7 cents lower than the piano's. So the step down, from 4:3 to 5:4, is a wide semitone: 111.7 cents, against the piano's 100. Let the note fall a little further than the piano would.

Where you hear it

The 4-3 suspension is one of the oldest moves in harmony: the fourth is held over from the chord before, clashes with the bass, then falls to the third. You hear it at cadences all through hymns and Bach chorales, often on the dominant just before the end: G sus4, G, then C.

In pop and rock, a sus4 often stands on its own, or swings back and forth with the major chord on the same root.

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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

Is sus the same as sus4?

Usually, yes: C sus on its own means C sus4, which is C, F and G.

What is the difference between sus2 and sus4?

Which note replaces the third: the second in a sus2, the fourth in a sus4. The two are close relatives: C sus4 has the same notes as F sus2.

Does a sus4 chord have to resolve?

In classical and hymn harmony it nearly always does, the fourth falling to the third. In pop and rock it often doesn't, and it's used for its open, unsettled sound.

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