Augmented 6th chords are chromatic chords that are built upon the interval of a major 3rd and an augmented 6th. They are very useful in modulation to different keys and have been used in various forms since the Renaissance period.
Augmented sixth chords are typically built on the flattened submediant (the 6th note of the scale)
They usually occur in minor key, but can be found in major keys through use of additional accidentals to “borrow” notes from the parallel minor scale.
Augmented 6th chords are dissonant and so “want” to resolve.
Typically, the lower note moves down a semitone and the upper note moves up a semitone to form an octave. If this is the case then the harmony usually leads to V (or sometimes Ic-V).
Types of Augmented 6th Chord
There are 3 common types of augmented sixth chords you will come across. All three of the chords include the major 3rd interval and the augmented 6th interval above the root note, but have differences in the other note added:
Italian Augmented 6th
French augmented 6th
German augmented 6th
The German sixth is the most commonly used of the three and works well as a pivot chord for modulation. The typical modulation you see is where the augmented 6th chord is built on the flattened submediant in the existing key and then leads to the dominant which becomes the new key. For example, in C the German sixth would naturally pivot the piece to G (the dominant).
However, the German sixth can also act as a dominant 7th chord of another key through enharmonic changes.
For example, in C, the German 6th can become dominant 7th of D flat if the F sharp is rewritten as a G flat. The distance in pitch remains the same, but the interval is expressed differently and so leads to the new key: