Augmented Sixth chords (+6) use the interval of an augmented sixth to resolve outwards to (typically) ^5 and dominant harmonies (cadential 6-4 included). Remember that augmented intervals will generally resolve outward.
The chord is built on the b^6 scale degree, with the inclusion of #^4 to form the augmented sixth interval. These two scale degrees can be thought of as leading tones from both above and below ^5.
The augmented sixth chord typically appears in one of three forms: Italian, German, and French.
- The Italian +6 is comprised of b^6, ^1, and #^4. It is the simplest (fewest chord tones) of the +6 varieties.
- The German +6 adds b^3 to the Italian version, resulting in b^6, ^1, b^3, #^4 (a major triad with an augmented sixth). Note that the German and Italian +6 chords are enharmonically equivalent to V7 chords. The spelling is important, as chordal sevenths resolve down by step, and augmented sixths resolve outward to the octave.
- The French +6 adds ^2 to the Italian +6, resulting in b^6, ^1, ^2, #^4.
Note that all varieties of the +6 chord include b^6 as the root, tonic, and #^4 (major third and augmented sixth intervals from the root).
Anecdotal evidence ascribes the Italian designation to the more usual simpler nature of Italian harmony (than German). It might also be found that Beethoven (a German) used the German variety more frequently. No one is quite sure. The French designation, however, is quite appropriate. The French +6 chord belongs to a whole-tone scale, the use of which began to develop with French music around the turn of the 19th to 20th century.
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