Music Theory Online: Augmented 6th Chords

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Augmented 6th Chords

An augmented 6th chord gets its name from the augmented 6th it contains. The chord is built on the flattened 6th degree (sub-mediant) of the major scale, or the 6th degree of a minor scale (their 6ths are already flattened).
In C major/minor, this would be A-flat. In C# major/minor, this would be A. etc.)

A standard ("Italian") augmented 6th chord contains:
1. The flattened 6th degree of the scale (root of the chord).
2. A major 3rd above the root.
3. An augmented 6th above the root.

In 4 part writing, the root would be doubled.

For C major/minor this would be:
italian augmented 6th chord

There are variants on the "Italian" augmented 6th, which are:

The French Augmented 6th:
Root [flattened mediant]
major 3rd above root
diminished 4th above root
augmented 6th above the root

french augmented sixth chord

The German Augmented 6th:
root [flattened mediant]
major 3rd above root
diminished 5th above root
augmented 6th above the root.

german augmented sixth chord

Resolving Augmented 6th Chords

Augmented 6th chords usually resolve "outwards" to the dominant chord:
The augmented 6th needs to resolve outwards and so does the augmented 4th:
Augmented 6th and augmented 4th resolve outwards
So the chord resolves to the dominant:
Augmented 6th chords resolve outwards to the dominant chord

The german augmented 6th must resolve differently, in order to avoid parallel 5ths that would occur between the rootand 5th when resolving straight to the dominant:

Resolving the german augmented 6th chord without parallel 5ths
So the chord usually resolves to Ic (or I 6/4) and then to V:

The german 6th Chord must resolve to Ic first

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