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Augmented sixth chords

Augmented sixth chords named after European nationalities include an augmented sixth, an enharmonic chord with a minor seventh. Notation traditionally uses the aug6 interval when voice leading creates both an upper and a lower leading note for the dominant. J. S. Bach himself employed the augmented sixth in his chromatic voice leading. The excerpt below is Die Kunst der Fugen from contrapunctus 3.

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In the classical romantic period, augmented sixth reached a more independent status. It is typical in cadences before a dominant chord or a fourth-sixth chord.

If thinking in piles of thirds, chords are inversions with chromatic changes. This is why the scale degree symbols are complex (see the example below), and chords were designated with names and symbols. An intriguing point is that the identity of the person behind the names is unknown. The terms "Italian 6th", "German 6th", and "French 6th" are, however, used internationally. Finnish also uses the term "English 6th", which in Walter Piston's Harmony (1987) is called the "Swiss 6th".

Bar 1 in the example below displays a typical diatonic progression towards the dominant in a minor key. When the progression becomes chromatic (b. 2), an augmented sixth appears between F and D sharp.

The degree symbol VI above bars 3-6 refers to the fact that the most common bottom note in augmented sixth chords is the leading note of the dominant (sixth degree in a minor scale, lowered sixth degree in a major scale). Augmented sixth chords can appear on the diminished second degree and resolve into the tonic; the symbol then would be, for example, IIG. In jazz theory, this chord corresponds to tritone substitution (see Functional harmony in jazz). The degree number is often left out, and the symbol is G6 instead of VIG.

Instead of a minor seventh, an augmented sixth is used for more distinct voice leading. On the other hand, musical tradition states that a dominant is never lowered (see Chromatic symbols).

The Italian sixth, It6, is a common three-note harmony in Viennese classicism, typically resolving into a dominant chord.

The German sixth, G6, most naturally resolves into a fourth-sixth chord in a cadence; in this example, it is enharmonic with the four-note chord F7.

The French sixth, F6, consists of interposed tritones and corresponds to the absolute chord F7b5.

The Swiss chord E6 only appears in major before a fourth-sixth chord in a cadence. It includes as many as three leading notes; the one leading to the diatonic third forms a rare double augmented fourth (2xy4) counted from the bass. It is an enharmonic equivalent of the German sixth, and some composers, for example Chopin, often write it in the more readable form G6.

An augmented sixth includes more chromatic signs in major than in minor since the sixth degree must be lowered. The example below compares augmented sixths with secondary dominant chords, close to them in sonority.

The raised fourth degree (#4) appears as an altered note in all secondary dominant chords. In augmented sixths, b6 is also present.

The German sixth in bar 4 resolves into the dominant and, in this context, creates a parallel motion by fifths. The term "Mozart fifth" is misleading; Mozart usually wrote a parallel fourth, correct according to the voice leading rules, instead of a parallel fifth.

Augmented sixth can be inverted (aug6 can become dim3), but the symbol remains the same.