The Augmented Sixth chord inherit the subdominant function of the IV degree chord. Yet the raised note makes it more like the secondary dominant of the V degree. We can see this very clearly in the French Sixth chord. We can analyze this chord as a secondary dominant of the V degree (B - D# - F# - A) with a lowered 5th (F natural in this case):

The German Sixth is enharmonically equivalent to a dominant 7th chord. Here you can see the German Sixth chord of A minor and the same chord transformed into the dominant 7th of Bb major:

We can also change a dominant chord into an augmented sixth. Here we see the C major dominant chord transformed into the German Sixth (G6) of B minor:

We can use these procedures to modulate to distant keys. Here we show the German Sixth of A minor used as the dominant chord of Bb major:

In this example, the dominant chord of C major is used as German Sixth of B major:


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José Rodríguez Alvira.