The augmented sixth chord inherits the subdominant function of the IV chord. The raised note makes it more akin to the secondary dominant of the fifth degree. This is very clear in the French sixth chord. This chord can be analysed as a secondary dominant of the V degree (B - D# - F# - A) with a lowered fifth (F natural in this case):

The German sixth chord is enharmonically equivalent to a dominant seventh chord. Below are the German sixth chord of A minor and the same chord transformed into the dominant seventh chord of B♭ major:

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

These procedures can be used to modulate to remote keys. Here, we demonstrate how to use the German sixth of A minor as the dominant chord of Bb major:

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