Augmented Sixth Chords
Augmented sixth chords are chromatic predominants named for the augmented sixth between ♭6̂ and ♯4̂ that resolves outward to the dominant; the three common types differ only in their added note.
The Italian augmented sixth (It⁺⁶) is the simplest of the three augmented-sixth chords. It contains exactly three distinct scale degrees: ♭6̂ (le) in the bass, 1̂ (do), and ♯4̂ (fi). The defining feature of all augmented-sixth chords is the interval of an augmented sixth between le and fi — both are active tones that resolve outward by half step to sol (5̂), the root of the dominant. Because the Italian sixth has only three pitches, one of them must be doubled in four-voice writing. The tonic (1̂) is the conventional doubling choice, giving the chord a stable interior while its outer voices drive toward V. In a standard progression the Italian sixth precedes a root-position V or V⁷, often with a cadential 6/4 inserted between them to smooth the voice leading.
The French augmented sixth (Fr⁺⁶) expands the Italian sixth by adding scale degree 2̂ (re), giving it four distinct pitches: ♭6̂–1̂–2̂–♯4̂. Like all augmented-sixth chords it is anchored by the interval of an augmented sixth between ♭6̂ (le) and ♯4̂ (fi), but the extra note — re — creates a tritone between 1̂ and ♯4̂ (or equivalently between 2̂ and ♭6̂), lending the French sixth a sharper, more dissonant character than the Italian. In four-voice writing each pitch appears exactly once; no doubling is needed. Voice leading follows the same outward resolution of le→sol and fi→sol that governs all augmented-sixth chords, while 2̂ typically steps up to 3̂ or holds as an inner voice and 1̂ descends by step into the dominant chord.
The German augmented sixth (Ger⁺⁶) also adds a fourth pitch to the Italian sixth, but instead of 2̂ it uses ♭3̂ (me): the full chord is ♭6̂–1̂–♭3̂–♯4̂. Enharmonically this stack of pitches is identical to a dominant-seventh chord (spell it with ♯2̂ instead of ♭3̂ and it reads like a V⁷ of the key a half step up), which gives the German sixth a richer, more lush sound than either the Italian or the French. The enharmonic resemblance to a dominant seventh is both the German sixth's greatest asset and its principal voice-leading challenge: resolving directly to V creates parallel fifths, so the common remedy is to interpose a cadential 6/4 (Ger⁺⁶ → I6/4 → V). The German sixth therefore often moves through a cadential 6/4 before reaching the dominant, though it can also resolve directly to V when the voice leading is managed — or when respelled (♭6̂–1̂–♯2̂–♯4̂) to reach a root-position V cleanly.
