Augmented sixth chord
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An augmented sixth chord is a chord containing the written interval of an augmented sixth between two of its notes. Although it had occasional use in Baroque music, it became a distinctive part of the musical style of the Classical period.
The reason an augmented sixth chord uses an augmented sixth rather than a minor seventh is contrapuntal. The raised sixth indicates its usual method of resolution: it is a leading-tone, much like the third of a dominant chord, which is meant to resolve by semitone upward. Though by itself an augmented sixth chord may sound similar (or identical) to a dominant seventh chord, its effect combined with the surrounding harmony is quite different.
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[edit] Italian sixth
The Italian sixth (
) appears frequently in Classical compositions in a minor mode, often as an extension of a half cadence appearing between IV6 and V. It contains three different pitches, taken from scale degrees ♭6, 1, and ♯4. In four part writing, 1 is usually doubled. Resolution of this chord usually proceeds to V, taking ♯4 up to 5, and ♭6 down to 5 (see example above). In the major mode, the chromatic effect is more pronounced, because while the minor scale already has a minor sixth degree (♭6), the major has a major 6, so the Italian sixth introduces two chromatically altered tones, rather than just one.
[edit] French sixth
The French sixth (
) is like the Italian, but with an additional tone on scale degree 2 (♭6, 1, 2, ♯4). This creates a rather unique sonority that is both inversionally symmetric, and transpositionally equivalent to itself at the tritone. Its use in Classical music is usually like the Italian sixth: resolving in the same way, but holding the new tone 2 in common with the next chord (see example above), and preference of one or the other usually has to do with the contrapuntal situation (the Italian is more common).
In the Romantic period, the French sixth gained more significance, as because of its equivalence to itself at the tritone it could be used not only to resolve to V, but also ♭II, the Neapolitan sixth. This provided an avenue for modulation to rather distant keys, which was a significant means of expression for Romantic composers.
The late Romantic composer Alexander Scriabin made very extensive use of the French sixth, exploiting its unique sound and its harmonic ambiguity in an extreme way.
Another wonderful resolution of the French is to the tonic. The Aquarium movement from Camille Saint-Saëns's The Carnival of the Animals is a striking example.
[edit] German sixth
The (
) is also like the Italian, but again with an added tone, this time on scale degree ♭3 (♭6, 1, ♭3, ♯4). It is enharmonically equivalent to a dominant seventh, specifically the applied dominant V of ♭II. Thus, like the French sixth, it was often used in this relationship by Romantic composers as a method of modulation to remote keys. In Classical music, however, it appears in much the same places as the Italian sixth, though it is less used probably because of the contrapuntal difficulties outlined below. Appropriately, it appears very frequently in the works of Beethoven, who was the most prominent composer during the transition between the Classical and Romantic periods.
It is difficult to avoid parallel fifths (see the lower two lines of the example above) when proceeding to V. This is one of the rare cases in counterpoint where parallel fifths are sometimes considered acceptable, though the German sixth is very often written with a suspension in order to avoid them.
Another way to avoid the parallel fifth problem is resolution of the German sixth to the tonic. This allows for two common notes for a minor tonic, or one common note for the major. A nice example of this is found in the high passage heard twice in "Passepied" from Debussy's "Suite Bergamasque". When resolving in this manner, it is also called an English augmented sixth, especially if it is spelled instead as ♭6, 1, ♯2, ♯4.
[edit] Other augmented sixth chords
The Italian, French, and German sixth chords are only the most common augmented sixth chords. All three may appear in inversion, especially the German chord. Wagner's Tristan chord is an unusual augmented sixth chord, and to an inventive composer there are many more possibilites.
In jazz, an augmented sixth chord would typically be described as a type of tritone substitution, and spelled as a dominant chord. Tritone substitutions are used more freely in jazz than augmented sixth chords are in the classical style; from a jazz perspective, the latter would be seen as a specific idiom for V7 of V, used mainly in the minor mode.
[edit] See also
[edit] References
Gauldin, Robert. Harmonic Practice in Tonal Music. W.W. Norton & Company, Inc., New York, 1997. ISBN 0-393-97074-4 (pp. 422-438.)
| Chords | ||
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| By Type | Triads | Major · Minor · Augmented · Diminished |
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| Sevenths | Major · Minor · Dominant · Diminished · Half-diminished · Minor-Major · Augmented major · Augmented minor | |
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| Extended | Ninth · Eleventh · Thirteenth | |
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| Other | Sixth · Augmented sixth · Suspended · Altered · Added tone · Polychord · Quartal and quintal · Tone cluster | |
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| By Function | Diatonic | Tonic · Dominant · Subdominant · Submediant |
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| Altered | Borrowed · Neopolitan sixth · Secondary dominant · Secondary subdominant | |
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