Three notes · 1 3 ♯5 · pure 16:20:25
The augmented chord
An augmented chord is two major thirds stacked: a root, a major third, and an augmented fifth 8 semitones up. It sounds stretched and unresolved, as if it's waiting for the next chord.
How it's built
It's a major chord with the fifth raised a semitone: C, E and G♯. On a piano every gap in it is a major third, including the one from G♯ up to the next C, so it splits the octave into three equal parts. That makes it symmetrical: C, E and G♯ augmented use the same three keys, and a piano has only four different augmented chords.
Singing it in tune
Switched to Pure, this page tunes it as two pure major thirds, 16:20:25. E sits 13.7 cents (a cent is a hundredth of a semitone) below the piano's, and G♯, a pure third above E, 27.4 cents below. Three pure major thirds fall 41 cents short of an octave, so the gap from G♯ up to the next C comes out wide: 427.4 cents against the piano's 400.
This is where pure and equal tuning disagree most. G♯ tuned as a third above E sits 41 cents lower than A♭ tuned pure as a minor sixth over C, though a piano plays both on one key. In practice the raised fifth is usually a moving note, rising a semitone into the next chord, so the line it's heading for matters as much as the chord.
Where you hear it
Most often between two chords, as one voice climbs by semitones: C, C augmented, then F or A minor, with G rising to G♯ and on to A. It's also the chord on the third note of the harmonic minor scale, E♭, G and B in C minor.
More chord types
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Questions
What notes are in a C augmented chord?
C, E and G♯. It's written G♯ rather than A♭ because it's the fifth of C, raised a semitone.
What does the + symbol mean?
Augmented: C+ and Caug are the same chord. In a chord symbol, + on its own means the fifth is raised.
What is the difference between augmented and diminished chords?
An augmented chord stacks two major thirds and raises the fifth; a diminished chord stacks two minor thirds and lowers it. Both are tense, but augmented sounds stretched and diminished sounds squeezed.
Why does an augmented chord sound unresolved?
Because it's symmetrical: its notes split the octave into three equal major thirds, which gives no note the clear feel of a root, so the ear can't tell where it's going. It usually resolves by one note moving a semitone.
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