Volume III · Chapter 22
Augmented Sixth Chords
MUSIC 21A · C-ID MUS 140Chapter quiz →
In the musicianship bookChapter 26: The Neapolitan and Augmented Sixths
Introduction: An Interval That Expands
Put ♭6̂ in the bass and ♯4̂ above it. The interval between them is an augmented sixth, and it pulls outward with tremendous force: ♭6̂ falls a half step, ♯4̂ rises a half step, and both land on 5̂ an octave apart. Chords built around this interval, called augmented sixth chords, are the most intense predominants in tonal music. They lead to V (or to the cadential six-four) and nowhere else.
For reasons nobody really knows, the three common types carry national nicknames: Italian, French, and German.
Click any example to hear it. ♭6̂ and ♯4̂ are shown in red.
Building the Three Types
All three share a core of three notes: ♭6̂ (in the bass), 1̂, and ♯4̂. The fourth note decides the type:
| Type | Scale degrees | A minor | C minor | C major |
|---|---|---|---|---|
| It+6 | ♭6̂, 1̂, ♯4̂ (double 1̂) | F – A – D♯ | A♭ – C – F♯ | A♭ – C – F♯ |
| Fr+6 | ♭6̂, 1̂, 2̂, ♯4̂ | F – A – D♯ – B | A♭ – C – F♯ – D | A♭ – C – F♯ – D |
| Ger+6 | ♭6̂, 1̂, ♭3̂, ♯4̂ | F – A – D♯ – C | A♭ – C – F♯ – E♭ | A♭ – C – F♯ – E♭ |
- The Italian has only three different notes; double 1̂ (never ♭6̂ or ♯4̂).
- The French adds 2̂, which creates a second tritone and an exotic, whole-tone sound.
- The German adds ♭3̂. It sounds exactly like a dominant seventh chord (in C: A♭–C–E♭–F♯ sounds like A♭7), which Chapter 24 will exploit.
- In a major key, ♭6̂ and ♭3̂ are borrowed through mixture, so the chords need extra accidentals.
Resolving Augmented Sixth Chords
- ♭6̂ falls and ♯4̂ rises, each by half step, to 5̂: the augmented sixth expands to an octave. Never double either note.
- The other voices move to the nearest tones of V: 1̂ falls to 7̂ (or holds into a six-four), 2̂ holds, ♭3̂ falls to 2̂.
- German to V creates parallel fifths (♭6̂–♭3̂ moving to 5̂–2̂). These are called "Mozart fifths," and you'll find them in real music, but in part writing avoid them: send the German to a cadential six-four first (♭3̂ holds or becomes ♮3̂).
- In a major key, when the German goes to I64, ♭3̂ moves up to ♮3̂, so it's often respelled as ♯2̂ (in C, D♯ instead of E♭).
Approaching Augmented Sixths
Augmented sixths usually come from a tonic or another predominant, and the bass often reaches ♭6̂ by step: from 1̂ (i–It+6, a leap down a third), from 4̂ (iv6–It+6, by half step), or from 6̂ in a descending line (the "lament bass" 1̂–♭7̂–♭6̂–5̂). The bass descent iv6–Ger+6–V is especially common, because the bass simply slides down 6̂–♭6̂–5̂ in major.
Try It: Augmented Sixth Builder
Augmented Sixth Builder
Choose a key and a type. ♭6̂ and ♯4̂ (red) expand outward to the octave on 5̂.
Summary
- Augmented sixth chords contain ♭6̂ (usually in the bass) and ♯4̂, which expand outward by half step to an octave on 5̂.
- Italian: ♭6̂–1̂–♯4̂ (double 1̂). French: adds 2̂. German: adds ♭3̂.
- They are predominants and lead to V or to the cadential six-four.
- Ger+6–V makes parallel fifths; resolve the German through I64 in part writing.
- The German sounds like a dominant seventh, a fact that opens the door to enharmonic modulation.