Augmented 6th Chords
Augmented Sixth chords, while sounding perhaps as exotic as the "Neapolitan" aka "Phrygian 6th" chord, are actually quite straightforward. An augmented sixth chord is a chord that contains the interval of augmented sixth—a claim no other chord can make. It is the existence of the augmented sixth within the spelling of the harmony that creates the distinctive Augmented 6th sound.
What is important here is, in fact, the sound. An augmented 6th is the same interval on the piano as a minor 7th, but spelled differently. The minor seventh that is featured in an F dominant seventh chord, for example, is spelled F-Eb, whereas the augmented sixth spelling of the same interval is F-D#. What makes the first interval a seventh are the notes F to E, whereas the second interval is a sixth due to the distance between F and D. The quality of the interval: Diminished, minor, major or augmented, depends on the chromatic alterations of the interval, as shown here in the second measure.
First, a clarification. Try very hard to make a distinction between the reason the "Neapolitan/Phrgian 6th" has the term sixth, and the reason the Augmented 6th chord has the term sixth:
- For the Neapolitan 6th, the "sixth" designation is due to the fact that the chord most often appears in first inversion.
- For the Augmented 6th, the "sixth" designation is due to the fact that there is the interval of an augmented sixth in the chord, not because of its inversion.
OK?
The Italian 6th
The basic gesture of the augmented 6th chord is a linear one. It takes a iv chord, originally in the minor key (here in C minor), places it in first inversion, then raised the original root of the chord a half step, to create the interval of an augmented sixth between the bottom note and the altered root:
The "It" doesn't mean that we call this an "it" chord, (like we don't have a better name for it, so we call it an "it.") Actually, "It" stands for "Italian," because there are in fact three types of Augmented 6th chords: The Italian, the German and the French.
Let's stick with the Italian for the moment.
An Italian sixth is an augmented sixth chord comprising just three notes— a bass note, the major third above, and the augmented sixth above that, as you see in the above example. In four part writing, the Ab tends to be doubled.
I've always imagined that this chord is called the Italian 6th because it grows out of a very vocal (i.e. linear) way of creating harmony, and likely began in Italian opera circles as an ornamentation. Rather than simple sing F to G in a iv-V passage, why not bring out the emotion by adding the the chromatic note inbetween?:
Above the word "my," on the second eighth note, an expressive augmented sixth has been created between the Ab and the F#. This emotional tug gives an even stronger sense of motion towards the dominant—an urging, almost—since the Ab clearly tends to fall towards the G, while the F# above is absolutely, inevitably rising towards the higher G. This arrival, on the doubled dominant, in dramatic, operatic fashion, through both wings of the stage, as it were (from the note above and the note below), was a powerfully expressive gesture employed by composers throughout the late 18th and 19th Centuries. Indeed, one of the most dramatic passages in all Classical music, the opening of Beethoven's 5th Symphony, reaches its first cadence through precisely this gesture:
You see in the cadence a tonic chord, followed by an Italian sixth chord (Ab, C, F#, abeit with the C in soprano) resolving to a V dominant chord.
This is what augmented sixth chords do: they set up a strong motion towards the dominant by creating a linear, dynamic tension between the notes of the augmented sixth. The Ab wants to pull dowards to the G, the F# wants to rise upwards to the G.
The German 6th
Now that you understand the essential gesture, the other two types of voicings, named for the other two dominant musical cultures of the 19th Century in Europe—Germany and France—evolved to help strengthen, harmonically, the fundamentally vocal genesis of the Italian 6th.
You'll notice that the original Italian 6th, if we consider Ab to be functioning temporarily as the root of the chord, lacks a perfect fifth above this root. This again points to the linear, vocal genesis of the chord. As this chord became a routine member of common practice harmonic language, especially in the music of the Viennese Classicists Haydn, Mozart and Beethoven, it proved to be helpful to the voice leading to add the fifth above the root. Thus, think of the German sixth as the Italian sixth to which an additional perfect fifth has been added. In the key of C minor, it is the Eb above the bass note Ab:
You can to see how chromatic this chord starts to become. Since augmented 6th chords, after being born in the minor mode, can also be used in the major mode for dramatic effect, suddenly this chord must be given three accidentals, leaving only the third, the C, as part of the original diatonic key, as shown in the second system above. What gives the chord away as a German sixth is the presence of the augmented 6th interval between the Ab and the F#. This is very important to recognize when analyzing Classical and Romantic era music!
The French 6th
I think this voicing is called he French 6th because there is a greater degree of chromatic coloration in the chord—a compositional approach especially loved by French composers. It is a highly unusual chordal structure, consisting, as you see in the first measure above, of two major thirds separated by a whole step (Ab - C ; D to F#).
Whereas the Italian and German sixths are created from the starting point of a iv chord in first inversion, the French sixth is created from the starting point of a ii half-diminished seventh chord in second inversion. The Ab (when in C minor) remains in the bass. the F remains in the top voice, and the third is present above the bass (the C, in this key) just as in the other "nationalities." But instead of the perfect fifth above the bass, there is an augmented fourth, the Ab to D.
This makes sense because the half diminished seventh chord, on the second degree of the minor scale, is a diminished triad plus a minor seventh. The existence of the diminished fifth translates into an augmented fourth when placed in second inversion. (D to Ab becomes Ab to D).
One of the characteristics of the French 6th is how this augmented fourth, the D, is in fact a common tone with the D of the dominant chord. This helps make for a smoother voice leading, especially when the Augmented sixth moves directly to V.
Here is the French sixth moving into a i6-4, V7, i cadence.
As with the German and Italian sixths, the French sixth can also appear in the major mode, making for a striking and dramatic voice leading.
Voice leading with Augmented 6th chords
A good way to get a feeling for Augmented 6th voice leading in four-part style is to start with the iv minor or ii half diminished seventh chord (depending on whether your moving to an Italian/German sixth or a French 6th) and place the Ab in the bass and the F on top (thinking in C minor). The voice leading would then look like this:
Notice how this voice leading allows each voice to have a relatively easy, singable line even while the harmony sounds rather chromatic. This is the trick for moving into and out of augmented 6th chords: to avoid awkward intervals such as having a voice sing from Eb to F#. THis is especially true of the alto, which is simply singing around an F minor triad. Such movement also avoids any parallel octaves by allowing the bass Ab to fall properly to the G and sending the Alto Ab up to the C of the cadential I six-four.
Sing through each voice while you listen to the soundfile, hearing your note within the harmony.