In this video, I’d like to show you what Augmented Sixth chords are.

Augmented Sixth chords are chords in first or second inversion, but with one altered note. Since the altered note transforms the major sixth interval (shown here between the soprano and bass parts) into an augmented sixth interval, such chords are called augmented sixth chords.  

These chords come in different varieties, each one characterized by the name of a country, like Italy, France or Germany. The traditional usage of an augmented sixth chord has been to employ it as a pre-dominant chord, in other words, it must progress to the dominant.

Let's look at the Italian Sixth first.

This is derived from a IV minor chord in first inversion. We know that IV minor is only found in a minor key, so the context is always a minor tonality, for example Cm. So, in Cm, the IV chord is Fm, and we write it in first inversion in order to have a sixth interval. Of course, the second inversion also contains a sixth interval, but the first inversion creates stronger root motion. The voicing of the IV minor chord is also critical, if we want to avoid parallel or hidden fifths, and the best solution is to have the 6th interval between the soprano and the bass parts.

Here is a chord progression containing an Italian sixth. See how the soprano is raised by a half step, to create the augmented sixth interval with the bass note. Notice that this sounds like a dominant seventh type chord, and if we respell the F# as a Gb, we can see from the Nomenclature panel that this is indeed an Ab dominant seventh chord with a double  3rd and no 5th. However, we cannot spell it this way, because then the seventh would be required to drop. On the other hand, the F# will to rise to the G in the V chord, which is what we want.

The next example is a French Sixth, in the key of Cm.

This is an augmented sixth chord derived from a ii minor 7 flat 5 chord in second inversion. Again, this is only found in minor keys, and again we voice the chord with the sixth interval between the bass and the soprano.

When we raise the F to create the augmented sixth interval, we can hear that it sounds like a dominant seventh type chord, but with a flat 5. If I respell the chord tones, you will see how it is related to an Ab7 chord.

However, we need to write the notes so that the F# rises to G, the D is tied with D, the C drops to B, and the Ab drops to G, thereby resolving to the notes of the dominant triad.

Our final example of a type of augmented sixth chord is the German Sixth.

A German Sixth chord is derived from a IV minor seventh chord in first inversion. This is usually applied in a minor key, but is frequently used in major keys as well. Listen to this example in Cm.

When using the German Sixth, a problem arises with voice leading. As you can see, parallel fifths are generated between the German Sixth and the dominant chord. The only way to resolve this problem is to delay the resolution to the dominant chord, by using an intermediate chord called the "cadential 64". This amounts to suspending the 3rd and the 5th of the dominant chord, and resolving them a beat later. As you can see, this resolves the problem.

Notice that by respelling the F#, the German Sixth is exactly the same as an Ab7 chord.

For resolving to a C major, we can respell the Eb as D#, so that it rises to the E natural in the cadential 64, in order to prepare the final resolution to C major.

I hope this has helped explain what augmented sixth chords are, and where they come from. Just remember that you can use them any time your composition contains predominant chords that are either IV minor, IVminor 7 or iim7b5. Also be sure to have the bass note moving from b6 to V, and keep the altered note in the soprano part.

Thanks for watching.