These chords either lead to or act like secondary dominant chords that, in effect, resolve to the dominant chord before the tonicized chord (*see note). For that reason, they are also referred to as pre-dominant chords.
If we consider the C major scale, these chords are built from the fourth degree (the subdominant) of the C minor parallel minor key – the Fm chord. When inverted, we get the notes Ab, C and F, leaving us a major sixth between the Ab and F – this inversion occurs due to voice-leading purposes. When the upper note of the chord is raised, we get an augmented sixth interval:
The reason for having an augmented sixth is to create a leading tone towards the dominant chord, causing the outer voices of the augmented sixth chord (Ab and F#) to resolve to the respective octaves of the G dominant chord.
Augmented sixth chords are not considered as being inverted chords, although they are originated from a chord inversion. That said, in this case, the root of the chord would be an Ab.
There are three main types of augmented sixth chords and these are:
- the Italian Sixth; being the only augmented sixth chord with three notes, it is enharmonically considered as an incomplete dominant seventh chord with only the root, third and minor seventh. This chord is spelled with an F# and an Ab because of its respective ascending and descending movement to the tonic of the dominant chord:
- the German Sixth; is the same as the Italian sixth but with an added note, the fifth above the root, that is missing from the Italian sixth chord. It sounds like a complete V7 chord – the Ab7:
- the French Sixth; also, the same as the Italian sixth but with an added note, the augmented fourth above the root – in this case, the D. It sounds like a V7 chord with a diminished fifth – the D7(b5):
If you notice, the German sixth and the French sixth chords are both dominant chords a tritone away. As seen with these chords, they share the same tritones (F# and C) and in relation to the dominant chord they resolve to (the G7), the D7(b5) can be considered as a secondary dominant, while the Ab7 can be considered as the secondary subV7.
In this sense, augmented sixth chords can be used much like you would use a “normal” secondary dominant that points to a tonicized chord. As a consequence, these chords are often used as means to a modulation because they are reinterpreted as the new dominant chord for the new key, acting as chromatic pivot chords.
For instance, the G7 chord in the key of C could become the German sixth in a different key and resolve to an F#7 that in turn would affirm the tonic B in the key of B major – we are modulating from C major to B major, a half-step below:
Or, instead of fulfilling its function as a pre-dominant chord, we can use the Ab7 as a primary dominant of the new key Db major, modulating to a key half-step above from the original:
The same could be done using the French sixth by passing it for a German sixth of a new dominant chord from a new tonality.
Again, it is all just a matter of reinterpreting the function of the involved chords and trick the listeners by suggesting you are going to somewhere only to break expectations – also see neapolitan chords.
* A tonicized chord is a chord other than the tonic chord to which a dominant seventh chord progresses and that temporarily makes it sound like a tonic. As seen, when this occurs, the used dominant seventh chord is called a secondary dominant.
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