Augmented sixth chord
This chord has its origins in the Renaissance,[2] was further developed in the Baroque, and became a distinctive part of the musical style of the Classical and Romantic periods.
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.
[4][5][6] 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.
This is the only augmented sixth chord comprising just three distinct notes; in four-part writing, the tonic pitch is doubled.
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.
[8] In the late Romantic period and other musical traditions, especially jazz, other harmonic possibilities of augmented sixth variants and sonorities outside its function as a predominant were explored, exploiting their particular properties.
Due to its construction of two tritones separated by a major third, it has transpositional invariance and is often used to create tonal ambiguity in highly chromatic music of the 19th century.
the prelude from Tristan und Ysolde and Bruckner's third symphony), but is most notable in Russian works such as Rimsky-Korsakov's Scheherezade[18] and Scriabin's Prometheus: The Poem of Fire.
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.
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[19] and Ravel's Scarbo.
Since there is no consensus among theorists that they are in root position in their normal form, the word "inversion" isn't necessarily accurate, but is found in some textbooks, nonetheless.
[citation needed] Sometimes, "inverted" augmented sixth chords occur as a product of voice leading.
[22] Seventeenth century instances of the augmented sixth with the sharp note in the bass are generally limited to German sources.
At the end of the second measure, the augmented sixth is inverted to create a diminished third or tenth between the bass and the soprano (C♯–E♭); these two voices resolve inward to an octave.
This is enharmonically equivalent to G♭–A♭–C, an incomplete dominant seventh A♭ 7, missing its fifth), which is a tritone substitute that resolves to G. Its inversion, A♭–C–F♯, is the Italian sixth chord that resolves to G. Classical harmonic theory would notate the tritone substitute as an augmented sixth chord on ♭2.
[27] The tendency of the interval of the augmented sixth to resolve outwards is therefore explained by the fact that the A♭, being a dissonant note, a diminished fifth above the root (D), and flatted, must fall, whilst the F♯ – being chromatically raised – must rise.
