The augmented 6th chords are predominant chords built on chromatic harmony for a smoother voice leading. They have many varieties depending on the requirements of the tonal music in different countries, out of which three are more common.
This article will cover their definition, chord spellings, types, main characteristics of the Italian, French and German versions, their notation, function, voice leading, and use.
Please read the complete article to understand Aug 6 chords and their functioning fully!
What is an Augmented 6th Chord (Aug6)
As per the music theory, an augmented sixth chord has an augmented sixth interval, usually between the bass note and the top note. It is not a usual chord built with a tertian harmony. As you know, a chord built on tertian harmony has intervals of major or minor thirds stacked over the root notes.
The main idea behind developing these chords is to have a smooth voice leading. Hence you may call them Augmented 6th Sonority instead of a chord.
An augmented sixth chord carries out the predominant function usually associated with the ii and the IV chords in a major scale and resolves to a dominant chord.
These chords were developed during the Renaissance era and got further refined during the Baroque period. They are one of the few types of chords that can be used to harmonize a non-diatonic note, along with other techniques like secondary functions, modulations, mode mixtures, and Neapolitan chords.
You can now familiarize yourself in the following sections with how the chords are formed and the music theory behind them.
The spelling of the Augmented 6th Chords
As stated above, they can be used to emphasize any note in the scale. Hence to build them, the first step is to pick a scale degree on which you want to focus them. Usually, it is the fifth scale degree to associate them with the predominant function and to have a better voice leading to the dominant chords.
However, the selected scale degree must be suitable to build a major or minor triad on it. This means that for major scales, you can select all scale degrees except the 7. Let us understand the whole process with an example.
Select the fifth-degree note G in the key of C major as your chosen scale degree to build the chord. The bass note for this augmented sixth chord is at an m2 interval or a half step above the selected degree, i.e., Ab. This note is the b6 note of the C major scale.
The top note is m2 below the selected degree one octave above. Hence, it is F# in the present case. This note is #4 in the selected major scale.
You will note that both are the upper and the lower leading tones to the selected scale degree. A leading tone is a pitch that resolves or leads to a note one-half step higher or lower.
Also, note that the interval between the bass note and the top note or the upper voice is an augmented sixth, which gives the chord its name. These pitches are at the flatted sixth scale degree (b6) and raised fourth degree (#4) of the scale for the selected scale degree 5.
The third voice of the chord is derived by selecting a note at an interval of M3 above the bass. In the present example, the third note will be a C or the degree 1 note. The three notes selected so far are Ab, C, and F# (b6, 1, #4).
These three notes form the core of any augmented sixth chord and are common for any type.
There is a fourth note which can be selected in three different ways forming the basis of the classification of the chords into,
- Italian augmented sixth chord,
- French augmented sixth chord, and
- German augmented sixth chord.
Types of Augmented Sixth Chords
You will know about each of these types of augmented sixth chords in detail in the sections below. It is thought by some that the selection of the fourth note is influenced by the type of music prevalent in these countries.
Italian Augmented Sixth Chords
The Italian sixth chord doubles the M3 note above the bass tone. So the four-note Italian augmented sixth chord has Ab C C F# in our example. As Operas were more prevalent in Italy, their music had a distinct bias toward melody with less emphasis on harmony. To keep the harmony simple, the M3 note is doubled. Thus the chord has only three distinct notes.
Note that M3 note C is the tonic of the scale.
You can also consider the Italian sixth as an incomplete version of the dominant 7th chord [Ab, C, Eb, Gb] with the omitted 5th. Some also call it a #iv6 chord as derived from the altered or raised 4th scale degree in the first inversion. Note that F is the 4th degree in our example and F# forms the altered 4th degree. F, Ab, and C form the F minor chord, the iv.
French Augmented Sixth Chords
The French sixth chords have an augmented 4th (A4) pitch above the bass note as their fourth voice in the chord. The added augmented interval in the chord results in a colorful but odd and unusual harmony, which is very characteristic of French music.
In your example above, this will a D at the second scale degree. All four notes in the French 6th chord in your example are [Ab, C, D, F#] or (b6, 1, 2, #4]. These are at intervals that are multiples of whole tones and form part of the whole-tone hexatonic scale.
The Ab7 chord has notes [Ab, C, Eb, Gb]. Note that it is enharmonically equivalent to this chord with a flat fifth tone or Ab7b5. It can also be considered as the second inversion of II7b5 [D F# Ab C].
German Augmented Sixth Chords
A German Sixth chord is constructed by adding a tone at the Perfect fifth interval above the bass as the fourth note of the chord. This is in line with the usual sturdy approach by the Germans.
In your example, this will result in Ab, C, Eb, and F# or [ b6, 1, b3, #4] as the four notes of your German Augmented Sixths harmony. This additional note Eb is at the flatted third scale degree (b3) of the C major. The notes of this chord are enharmonically equivalent to the dominant seventh chord, Ab7, but it does not function in the same way as Ab7 is not a diatonic chord.
The Aug 6 chord found a lot of usage in ragtime music and the compositions of Beethoven.
There are other variations of the Aug 6 chords, like the Australian sixth or the Japanese sixth, but they are not common.
Notation of the Augmented Sixth Chords
These chords are represented by their abbreviated type, followed by +6 and a forward slash with the initially selected scale degree after the slash symbol. Hence,
- Italian 6th is represented as It+6/(5) or It+6/(1), where (5) and (1) represent the selected scale degree upon which these chords are built. If the selected degree is 5, you may not use the slash symbol and just notate it as It+6. For any other selection, it is mandatory to indicate the degree.
- French 6th – Fr+6/(5) or Fr+6/(1). Balance rules are the same as above.
- German 6th – Ger+6/(5) or Ger+6/(1).
You may use the figured bass notation for inversions.
Augmented Sixth Chord in Minor Scale.
So far, you saw the example of major keys where the core notes have degrees b6, #4, and 1. The fourth note belongs to degrees 1, 2, and b3 in the Italian, French and German versions.
In minor keys, Ab is a diatonic note at degree 6. In addition, Eb is also in the key at degree 3. Hence the Aug 6 chords are more common in the minor mode. In a major mode, the b6 is borrowed from the parallel minor mode.
Examples of the Augmented Sixth Chords
Let us consider the scale degree 1 (C) of the C Major key to find out the notes of the Aug6 chords.
- The bass note is a half step above the selected note C = Db.
- The top note is m2 below C one octave above = B
- The third core note is M3 above bass = F, making the core chord with notes Db, F, and B
- It+6/(1) = [Db F F B].
- Fr+6/(1) = [Db F G B].
- Ger+6/(1) = [Db F Ab B].
Use of the Augmented Sixth Chords
Let us now focus on the different functions the Aug 6 chords can serve in your music, along with some sample harmonic progressions and their voice leading.
When built based on degree 5, the augmented sixth chords precede the V chord, I6/4 or i6/4. As stated earlier, these chords, on degree 5, function as predominant chords and resolve to a dominant chord. Here I6/4 or i6/4 functions as a dominant chord (Cadential6/4). Let us see them in action in this role in the harmonic progressions below.
Harmonic Progressions with V chord & Voice Leading
Usually, the #4 of the augmented sixth resolves outward while the bass note
The Ger+6/(5) or simply Ger+6 goes to I6/4 or i6/4 and not to the root position dominant chord V directly. Let us see the voice leading of Ger+6 to I6/4 in the staff diagram below.
The diagram below shows the voice leading if we try to directly resolve the Ger+6 chords to V. Notice that the Bass and the Alto voices are moving in the parallel fifths, which is not allowed in the voice leading. Hence you must first go to I6/4 and then to V.
The French and Italian sixth chords do not have Perfect 5th above bass as any of their note, so they do not face this issue.
Let us look at other progressions,
+6/(5) – V4/3/V – V6 – i, or
+6/(5) – viio6/5/V – V6 – i
Here +6 denotes that it can be either of the three varieties. In both these progressions, the Aug6 chord leads resolve to secondary function chords before the progression moves to the first inversion of the dominant chords and the tonic triad.
Consider the C minor key with three flats in its key signature. The core of the chord is Ab, C, and F# discussed above. G major is the V chord for the key. V/V, or the secondary dominant of the V chord, is D. V4/3/V denotes the D chord in its second inversion, [A, C, D, F#]. The V6 is G major in its first inversion leading to the tonic chord i. This is shown in the diagram below.
Use as a Neighbor Chord for V
The Aug6 chords based on degree 5 can be used as a neighbor chord for V, as shown in the diagram below.
Other Functions of Aug 6 Chords
In the discussion so far, we have seen that the Aug6 chords built on scale degree 5 are functioning as the predominant chords with chromatic alteration and ultimately leading to the V chord.
Since they precede the dominant or the tonic in the second inversion, they can also be referred to as chromatic passing chords, with tones b6 and #4 being the chromatic passing tones.
These chords can be used to harmonize any non-diatonic note by appropriately selecting the scale degree in their formation. For example, the chords built on b2 as the bass will resolve to I, and if you take the 4th scale degree as the bass, the chord resolves to I or I6.
As stated above, the It+6 is an incomplete dominant 7th, Fr+6 a 7b5, and Ger+6 an enharmonically equivalent complete dominant chord built on chromatic notes as their bass notes. Hence they can be used to tonicize the remotely related keys or for chromatic modulations. Such modulations are known as enharmonic modulations. You can see a completely worked-out example with the Aug 6 chords in our article on modulation music theory.
Inversions Of the Augmented Sixth Chords
The term inversion is normally not used for the Augmented sixth chords, as there is a lack of consensus to call their normal bass position the root.
We hope that you have found our article on augmented sixth chords useful and informative. If you want to have any clarification on the above topics or want to provide us with your experience with these chords, you can comment below. We will be happy to respond.