What are augmented sixth chords?

Answer 1

Augmented sixth chords can be scary because they are typically the first non-triadic chord a student runs into. To understand how they work, you have to set aside your understanding of triads and think about the chord from a purely voice leading perspective.

We know from triadic harmony that half steps can create a sense of expectation when they resolve to their proper note. The augmented sixth utilizes this principal to an extreme degree. In the key of C, we want a chord that very strongly leads into our Dominant, the G chord (in other words, we want a really powerful Predominant chord). Thinking purely from a voice leading perspective, the most powerful thing we could do harmonically to lead into G would be to have both F#, a half step, and Ab, another half step, both resolve up and down respectively. That is the basis for our augmented sixth chord.

So we have our Ab (traditionally as the bass note) and our F#, but we need some more notes to fill out that sound! Well, composers tried a few. Adding the tonic makes sense, since it's a pretty important note, so we'll call that the Italian augmented sixth. When we add the second above the tonic, we'll call it French, and when we add the minor 3rd, we'll call it the German augmented sixth. Don't take those names too literally, as tradition is really the only reason why we still use them.

Answer 2

An augmented sixth chord is a type of non-diatonic chord that traditionally serves a predominant function and resolves to a dominant chord such as V or V⁷. The augmented sixth chord is often preceded by another predominant or tonic chord. All augmented sixth chords contain the scale degrees 1, #4, and b6. The #4 and b6 create tension that resolves in the V. The #4 and the b6 want to resolve to scale degree 5, with the #4 moving up and the b6 moving down.

Augmented sixth chords are usually found in first inversion, with the b6 in the bass position and the #4 above (usually in the soprano voice). This creates an augmented sixth interval in the chord, which is where we get the name of this chord.

There are three main types of augmented sixth chords: Italian (It⁺⁶), French (Fr⁺⁶), and German (Ger⁺⁶).

The Italian augmented sixth chord adds the first scale degree (this is the pitch that is doubled in four-part writing). The French augmented sixth chord adds the second scale degree (2). The German augmented sixth chord adds a lowered third scale degree (b3).

For example, in the key of C major, of which the dominant is G, the notes of the Italian augmented sixth chord would include Ab (b6), C (1), C (1), and F# (#4). The French augmented sixth chord would include Ab (b6), C (1), D (2), and F# (#4). The German augmented sixth chord would include Ab (b6), C (1), Eb (b3), and F# (#4).