Augmented Sixth Chords
There are at least three types of augmented sixths: the Italian sixth, the French sixth, and the German sixth. Those chords are generally used to reach the dominant or tonic chord in second inversion when performing a cadence in major and minor keys.
The Italian Sixth
The Italian Sixth chord is formed on the fourth degree. Most commonly, it is in first inversion. Its root is raised creating an augmented sixth interval with the bass:
Italian Sixth, A minor
Italian Sixth, A Major
Note: In the case of major keys, the third of the chord should also be altered so that it will have the same alterations as in the minor key. Therefore, the chord is said to have been borrowed from the minor key. This applies also to the other chords of the augmented sixth and to the Neapolitan Sixth.
The French Sixth
The French sixth is formed on the second degree. It is a seventh chord. Most commonly, it stands in its second inversion. Its third is raised in order to build an augmented sixth interval with the bass:
French Sixth, A minor
French Sixth, A Major
The German Sixth
The German sixth chord is built on the fourth degree. It is a seventh chord. It is most commonly in the first inversion. Its root is raised in order to create an augmented sixth interval with the bass:
To learn more about augmented sixth chords click here.