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I personally think that augmented chords can be just as consonant as major if used in a certain way. For example, in the first octave of a piano, if I play an augmented chord, yes it will want to resolve but not as much as say if I played that same chord an octave lower in the small octave. But in a way, augmented is ambiguous. You don't have 1 single root, you have 3 roots, 3 mediants, and 3 dominants. But at the same time these can be viewed as inversions of a single chord.

Here are the resolutions I get for C augmented in order of most natural to least:

C aug -> A minor 1st inversion(Harmonic minor scale has this augmented chord in it)

C aug -> C major

C aug 1st inversion -> E major

C aug 1st inversion -> A minor 2nd inversion

C aug 1st inversion -> C major 1st inversion

C aug 1st inversion -> C# minor 1st inversion

C aug 2nd inversion -> Ab major

C aug 2nd inversion -> A minor

C aug 2nd inversion -> C major 2nd inversion

C aug 2nd inversion -> F minor 1st inversion

That's all from 1 chord but that is 3 separate major/minor key pairs. At the same time E major can be viewed as the resolution of E aug and same for the Ab major and Ab aug. In turn C major can be viewed from any of those 3 keys as a resolution so what key are you playing something in if you are using the augmented chord as a dominant function chord? The major with the same lower note as the chord? The relative minor of that? Something else entirely? It sounds like it is not sure where it wants to resolve, just where it naturally would land out of 6 chords(namely the relative minor of the lower note). But all 6 chords are resolutions that sound equally as good.

With diminished you have a similar thing going on but only between 2 chords which are not equidistant on the circle of 5ths(for example B major and C minor both from C diminished). I mean to go from B major to C minor would take 8 or 4 steps through the majors and then 1 more step to relative minor depending on whether you go in the flat direction or in the sharp direction. With augmented, it is 4 steps in either direction through the majors that both resolve so what key are you in? C major? E major? Ab major?

So how can you tell the key a piece is in just from an augmented chord in the harmony? Is that even possible?

  • Just like with diminished 7th chords, I suspect that the tonic is whatever the root of the chord straight after the (augmented/diminished) dominant-function chord is. (I've used augmented chords as dominant-function chords before, and I've also found such chords in the Prism Plains theme from Kirby Squeak Squad.) – Dekkadeci Aug 20 '18 at 23:37
  • Since there are only really four augmented chords in equal temperament tuning (C C# D and D#) if you have four resolutions for the "first and second inversions" there are also four for the "root position" (I use the quote marks because there is no real difference between the "inversions" and "root" augmented chords - Caug first inversion and Eaug root position are identical). One of the missing resolutions is Caug -> F which was quite common in the classical period. – user19146 Aug 21 '18 at 7:31
  • Actually there are (at least) SIX resolutions of an augmented chord that all work the same way: just move one of the notes either up a semitone or down a semitone. But maybe the OP didn't consider "Caug -> C#minor" to be "in the key of C major" (though I would consider it that way - C#minor is just the minor form of the "Neapolitan" chord, more usually spelled as D flat not C sharp). – user19146 Aug 21 '18 at 7:37
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Starting in the 19th century, augmented triads started to be used as dominant sonorities. Or, put another way, augmented triads became acceptable dominant sonorities in the 19th century.

In other words, augmented triads can be the dominant of the tonality. C augmented (C E G♯), therefore, would be the dominant of F (major or minor), and thus you might be in the key of F.

If this C augmented is spelled enharmonically as E augmented (the chord thus being E G♯ B♯), you would resolve to A. If the C augmented is spelled as A♭ C E/G♯ B♯ D𝄪, it would resolve to a tonic of D♭ or C♯, respectively.

And for what it's worth, the III+ augmented chord (the augmented triad in the harmonic scale) is very rarely used in tonal music. In my experience, a III+ chord is usually a V chord with scale-degree 3 functioning as a non-chord tone.

  • Would C augmented be spelled with Ab, as then there would be no augmented 5th, only a minor 6th? Or maybe you meant 'the chord containing these notes'. – Tim Aug 21 '18 at 10:38
  • @Tim No, I meant the correct tertian spellings of those augmented triads. I'll edit to try and make it more clear. – Richard Aug 21 '18 at 14:35
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Rather like the diminished chords, of which there are essentially three, after which, going up a semitone each time, we get back to the same notes as the first one; there are four augmented chords.

Their names may be different, and the names of their notes certainly will be, but essentially their make-up results in only four.

For example, C E G# =C+, But E G# B# = E+, and G# B# Dx = G#+. The last could also be spelled Ab C E. So there's 3 different keys to head to after hearing those three notes, which constitute a kind of dominant chord. Their inversions may feel the need to head in a particular direction, but that's subjective.

It stands to reason that there's C#+, D+, and Eb+, going up in semitones, before we hit the same notes again as the first example, C+.

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Same key as it would if the dominant was a plain major triad.

You're over-thinking this. C7 is the dominant of F. C7(#5) is still the dominant of F. Maybe even stronger because as well as the E - Bb tritone pushibg towards F - A, we also have the G# pushing towards A. (They might not both actually resolve to A, but that's another matter.)

And a chromatic note doesn't have to imply a change of key. (Otherwise there wouldn't BE any chromatic notes, just loads of modulations, sometimes to several keys at once!)

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