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Augmented Sixth Chord

Augmented Sixth Chords come in three forms: German, French and Italian.

All three forms of Augmented Sixth Chords function as a sub-dominant (subdominant) in that they precede and in fact "tee-up" the dominant chord. They also may be considered altered versions of secondary dominants.

The French Augmented Sixth Chord consists of a major third built upon the sixth scale degree in a minor key or the flat sixth scale degree in a major key. In addition to the major third, the interval of an augmented sixth above the bass note and a major second above the bass note must be constructed.

Consider a second inversion secondary dominant seventh chord which tonicizes the dominant. This chord (sometimes referred to as a "double dominant" or a V43 of V can easily be molded into a French Augmented Sixth Chord simply by lowering its bass note by a minor second. To find the theoretical root of the French Augmented Sixth Chord, simply stack its notes in thirds to find that the root is in fact the second scale degree. This same scale degree is the root of the secondary dominant of V and further explains why the French Augmented Sixth Chord functions as a sub-dominant chord.

The German Augmented Sixth Chord consists of a major triad built upon the sixth scale degree in a minor key or the flat sixth scale degree in a major key. In addition to the major triad, an additional note is used which is an augmented sixth above the root os the triad.

Theoretically, the root of the German Augmented Sixth Chord is not the bass note but rather the sharp fourth scale degree because when the notes of the chord are stacked in thirds the sharp fourth scale degree falls to the bottom. In this position, the theoretical third of the chord is the flat sixth scale degree. Since the third of the chord appears in the bass, the German Augmented Sixth Chord is theoretically in first inversion and may also be considered an altered secondary dominant in the following way: If the bass note of a VII65 of V is lowered by a minor second a German Augmented Sixth Chord appears. Therefore the German Augmented Sixth Chord may be viewed as an altered VII65 of V.

The Italian Augmented Sixth Chord may be considered an altered sub-dominant. In a minor key, if the root of a second inversion subdominant chord is raised by a chromatic half step, the resulting three pitches form an Italian Augmented Sixth Chord. In this case, note that there is the interval of an augmented sixth between the bass note and one of the upper notes.

Note that every augmented sixth chord has the interval of an augmented sixth in it. The rest is just dressing. (German, French and Italian Dressing.)

When this most interesting interval resolves, it always resolves out to an octave. For example, in the key of G minor, the interval of an augemented sixth would occur between the bass note Eb and an upper note C#. The Eb will always move to D while the C# also moves to D but in a different octave. The harmonization of this octave is often a cadential 6-4. However, in the case of the French and Italian chords, the harmonization may also be a V or V7.

There are many other explanations of the derivations of Augmented Sixth Chords. Consider that in a major key, the Italian is a VII of V chord with its third lowered by a chromatic half-step. Also in a minor key, the German may be considered a V43 of V with its second lowered by a chromatic half-step.

-Dr. Jack
28 May 2008