In music theory, an augmented sixth chord contains the interval of an augmented sixth, usually above its bass tone. This chord has its origins in the Renaissance, was further developed in the Baroque, and became a distinctive part of the musical style of the Classical and Romantic periods.
Conventionally used with a predominant function (resolving to the dominant), the three most common types of augmented sixth chords are usually called the Italian sixth, the French sixth, and the German sixth.
Augmented sixth interval
The augmented sixth interval is typically between the sixth degree of the minor scale, , and the raised fourth degree, . With standard voice leading, the chord is followed directly or indirectly by some form of the dominant chord, in which both and have resolved to the fifth scale degree, . This tendency to resolve outwards to is why the interval is spelled as an augmented sixth, rather than enharmonically as a minor seventh ( and ).
Although augmented sixth chords are more common in the minor mode, they are also used in the major mode by borrowing of the parallel minor scale.
Types
There are three main types of augmented sixth chords, commonly known as the Italian sixth, the French sixth, and the German sixth.
Though each is named after a European nationality, theorists disagree on their precise origins and have struggled for centuries to define their roots, and fit them into conventional harmonic theory. According to Kostka and Payne, the other two terms are similar to the Italian sixth, which, "has no historical authenticity-[being] simply a convenient and traditional label."
Italian sixth
The Italian sixth (It<sup>+6</sup> or It<sup>6</sup> or iv<sup>6</sup>) is derived from iv<sup>6</sup> with an altered fourth scale degree, . This is the only augmented sixth chord comprising just three distinct notes; in four-part writing, the tonic pitch is doubled<!--wouldn't it be because it's the best voice leading strategy?-->.
The Italian sixth is enharmonically equivalent to an incomplete dominant seventh. VI7=V7: A, C, (E,) G.
Its label in functional theory is D/5-_7.
French sixth
thumb|350px|A French sixth chord in Schubert's [[Die schöne Müllerin, #5: "Am Feierabend" ]]
The French sixth (Fr<sup>+6</sup> or Fr) is similar to the Italian, but with an additional tone, . The notes of the French sixth chord are all contained within the same whole tone scale, lending a sonority common to French music in the 19th century (especially associated with Impressionist music), though they also make frequent appearances in Russian music.
This chord has the same notes as a dominant seventh flat five chord and is in fact the second inversion of II<sup>75</sup>.
Its label in functional theory is D5-_7.
|The chord can resolve to a chord, functionally either as a cadential intensification of V, or as the second inversion of I. The cadential , in turn, resolves to a root-position V. This progression ensures that, in its voice leading, each pair of voices moves either by oblique motion or contrary motion and avoids parallel motion altogether. In minor modes, both and do not move during the resolution of the German sixth to the cadential .
In major modes, 3 can be enharmonically respelled as , allowing it to resolve upwards to . This may be called a doubly-augmented sixth, although in reality it is the fourth that is doubly augmented. Such anomalies usually have alternative interpretations.
Function
Standard function
From the Baroque to the Romantic periods, augmented sixth chords had the same harmonic function: as a chromatically altered predominant chord (typically, an alteration of ii, IV, vi<sup>7</sup> or their parallel equivalents in the minor mode) leading to a dominant chord. This movement to the dominant is heightened by the semitonal resolution to from above and below (from and ); essentially, these two notes act as leading-tones.
During the Baroque and early Classical periods, for instruments tuned to meantone systems rather than well temperaments, the augmented sixth note (6) produced an excellent approximation to a harmonic seventh. The match is particularly close in quarter comma meantone, where 6 is only 3 cents flat from H. This made a major triad with an added 6 a fully consonant / harmonic chord (harmonics 4, 5, 6, 7); as opposed to a modern equal tempered dominant seventh chord (M add7) which misses being harmonic with the minor 7th pitch 31 cents sharp – a dissonance.
This characteristic has led many analysts to compare the voice leading of augmented sixth chords to the secondary dominant V of V because of the presence of , the leading-tone of V, in both chords. In the major mode, the chromatic voice leading is more pronounced because of the presence of two chromatically altered notes, and , rather than just .
In most occasions, the augmented-sixth chords precede either the dominant, or the tonic in second inversion. The augmented sixths can be treated as chromatically altered passing chords.