An augmented sixth chord contains the interval of an augmented 6th. The four types of Augmented Sixth chords (Italian, German, Enharmonic German and French) are all altered chords. They usually function as preparations to the dominant (the V chord), and can be used to replace IV or II as well. Each Augmented Sixth Chord contains the tonic note, a raised 4th scale degree and a lowered 6th scale degree (both surrounding and leaning towards the 5th scale degree, the dominant note). In both major and minor keys an accidental is required for the raised 4th scale degree; And in Major keys another accidental is required for the lowered 6th scale degree.
The Scale Degrees of the Augmented Sixth Chords are:
Italian: #4, b6, 1
German: #4, b6, 1, b3
French: #4, b6, 1, 2
Enharmonic German: #4, b6, 1, #2 (Enharmonic German is the same thing as the German except you use a #2 instead of a b3).
These chords can be used in any inversion. In the Italian chord, the tonic is doubled.
How to resolve them? These chords want to go to either the V, V7, or I64. They function much like secondary dominants. In the Italian Chord: The #4 goes up to the 5th scale degree, the b6 goes down to the 5th scale degree, the first scale degree goes down to the 7th scale degree (or raised 7th in minor keys), and the doubled first scale degree goes up to the 2nd scale degree. The result is: Viola, the V chord. The French chord adds a 2nd scale degree to the mix (instead of the extra tonic), which stays where it is. Voila, the V chord. The German chord adds a b3 scale degree to the mix (instead of the extra tonic), which resolves downwards to the 2nd scale degree. Viola, the V chord. When resolving the German chord to V, Parallel 5ths are the result. The only way to get around this is to change it to a French (b3 goes to 2) or Italian Chord (b3 goes to 1) before the resolution.
When resolving the Italian chord to a I64, The #4 goes up to the 5th scale degree, the b6 goes down to the 5th scale degree, One of The Tonics stays put and the other tonic skips up to the 3rd scale degree. When resolving the French chord to a I64, The #4 goes up to the 5th scale degree, the b6 goes down to the 5th scale degree, The Tonic Stays put, and the 2nd scale degree steps up to the 3rd scale degree. When resolving the German chord to a I64, The #4 goes up to the 5th scale degree, the b6 goes down to the 5th scale degree, the b3 goes up to the 3rd scale degree in major or stays put in minor, and the Tonic Stays put. When augmented sixth chords resolve to V7, the raised 4th scale degree often resolves downward (rather then the expected upward) by a halfstep, to form the 7th of the V7.
Augmented 6th chords may be used as a Dominant, replacing V. When this is done it is best viewed simply as V7 with a b5 (V7b5). Augmented 6th chords may also be used as a prepartion to a secondary dominant, leading to secondary dominants (this is the coolest part for me)! You could use an Italian Augmented 6th of V/V, a German Augmented 6th of V/II, a French Augmented 6th of V/III, etc. It resolves to a secondary dominant in the same way it normally resolves. This can create some amazing tension and movement.
A noteable thing about these chords is that they create the sound of altered dominants. The Italian Chord can be thought of as a dominant7b5 (without the root). The French Chord can be thought of as dominant7b5.
The German chord can be thought of as a dominant7b5b9 (without the root) (wow is that an awesome dark sound or what?).
Play around with these. They are awesome sounds.
