What key is a piece in if an augmented chord is used as a dominant?

Question

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?

Answer 1

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.

Answer 2

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+.

Answer 3

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!)

Answer 4

It would still be the same key as if the dominant was a major triad or a dominant-seventh (major-minor seventh) chord.

---

An augmented dominant chord is an altered chord that is derived from the V chord of a key. The 5th note of the chord is raised, causing it to become an augmented triad. We need to remember that an augmented chord is considered dissonant, therefore it requires a specific way of resolution. The 3rd and the 5th of the chord need to move a half-step (minor 2nd) up, and the resolved chord must always be a major triad.

We have a set of a dominant and secondary dominants in all 30 keys. In C-major, for instance, we would have:

• G - C (C: V - I)
• A - Dm (C: V/ii - ii)
• B - Em (C: V/iii - iii)
• C - F (C: V/IV - IV) (same as C: I - IV)
• D - G (C: V/V - V)
• E - Am (C: V/vi - vi)
• F♯ - Bdim (C: V/vii^o - vii^o)

We can turn all of them to augmented chords. We get:

• Gaug - C (C: V⁺ - I)
• Aaug - D (C: V⁺/ii - V/V)
• Baug - E (C: V⁺/iii - V/vi)
• Caug - F (C: V⁺/IV - IV)
• Daug - G (C: V⁺/V - V)
• Eaug - A (C: V⁺/vi - V/ii)
• F♯aug - B (C: V⁺/vii^o - V/iii)

Please note that the secondary V⁺ chords over ii, iii, vi, and vii^o are being resolved to another secondary dominant because we need to resolve the 3rd and the 5th altogether.