Augmented sixth chords

These are a group of chromatic chords usually occur on the flattened submediant or flattened supertonic. In harmonic progression, their place is invariably before the dominant (V) or the cadential six-four (Ic). The augmented sixth chord resolves by opening outward to form an octave, and a dominant chord. If the augmented sixth is on the flattened submediant (AF# in C major), the resolution will be on the dominant seventh chord (G in C major). If it is on the flattened supertonic (Db:B in C major), the resolution will be on the dominant seventh chord on the tonic (one way of
modulating to the subdominant). The German sixth is enharmonically equivalent to the dominant seventh. The drive of the (German) augmented sixth chord towards the dominant is frequently exploited in harmony (used instead of II). They are also used to modulate by forming a German sixth on the flattened submediant of the target key (see modulation).