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,
vi7 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.
Other functions {{Image frame|content= { \new PianoStaff 1^\markup { "Italian" } ^\markup { "French" } \voiceOne ^\markup { "German" } } \new Voice \relative c'' { \clef treble \key c \major \time 4/4 s1 s s s \voiceTwo aes2 g g1 } >> \new Staff > >> } |width=445|caption=Augmented sixths as dominants in C major, according to Tchaikovsky. Notice the early resolution of an inner voice to avoid
parallel fifths in the last example. and
Scriabin's Prometheus: The Poem of Fire. The chord is separated by only a single note from the half-diminished chord, or the "Tristan chord," as well as the German sixth or dominant seventh. Tonal ambiguity is created by the French sixth as it is symmetrical about a tritone, for example, the notes of a French sixth chord built on G are the same as the notes as the chord built on C sharp, up to enharmonic equivalence. Due to this tonal ambiguity, the French sixth is often used in lieu of the triad and carries with it an unresolved and uneasy sound. Scriabin also began to add chord extensions to the French sixth, for example, he added a sixth and a ninth to create his 'mystic chord' which is found in his aforementioned
Prometheus tone poem. The chord is usually combined with the
octatonic, or diminished, scale, as the scale contains two distinct French sixths and thus has similar symmetric properties. This combination can be found ubiquitously in much of Rimsky and Scriabin's music, as well as in some 20th century French works such as
Debussy's Nuages and
Ravel's Scarbo.
Tchaikovsky considered the augmented sixth chords to be
altered dominant chords. He described the augmented sixth chords to be
inversions of the
diminished triad and of dominant and
diminished seventh chords with a lowered second degree (), and accordingly resolving into the tonic. He notes that, "some theorists insist upon [augmented sixth chord's] resolution not into the tonic but into the dominant triad, and regard them as being erected not on the altered 2nd degree, but on the altered 6th degree in major and on the natural 6th degree in minor", yet calls this view, "fallacious", insisting that a, "chord of the augmented sixth on the 6th degree is nothing else than a
modulatory degression into the key of the dominant". The example below shows the last nine measures from
Schubert's
Piano Sonata in A major, D. 959. In m. 352, an Italian sixth chord built on scale degree functions as a
substitute for the dominant. {{block indent| { #(set-global-staff-size 18) \new PianoStaff \ottava #0 r r2\fermata r2 \tuplet 3/2 { bes,,8 d f } \tuplet 3/2 { bes d f } bes8 r \ottava #1 \ottava #0 r r2\fermata \break r2 \tuplet 3/2 { a,,8 cis e } \tuplet 3/2 { a cis e } \tuplet 3/2 { a cis e } \tuplet 3/2 { cis a e } \tuplet 3/2 { a_\markup { \italic "dimin." } e cis } \tuplet 3/2 { a e cis } R1 r2 4 r 1 } \new Voice \relative c { \clef bass \key a \major \time 4/4 \once \stemDown 4\pp } >> \new Staff 8 r r2\fermata \clef bass \tuplet 3/2 { bes,,,8 d f } \tuplet 3/2 { bes d f } r2 \clef treble r4 8 r r2\fermata \clef bass \tuplet 3/2 { a,,,8 cis e } \tuplet 3/2 { a cis e } r2 R1 \tuplet 3/2 { a8 cis e } \tuplet 3/2 { cis a e } \tuplet 3/2 { a e cis } \tuplet 3/2 { a e cis } a4 r r 1 \bar "|." } >> >> } }} ==Inversions==