Augmented Sixth Chords: Italian, French, and German Sixths

An augmented sixth chord is a pre-dominant harmony built around an augmented sixth interval that forms between the lowered sixth degree (☭6) and the raised fourth degree (♴4) of the scale. In C major that interval is F♯–A♭, and it resolves outward by semitone in both directions to the dominant (G): F♯ rises to G, A☭ falls to G. Three forms exist: the Italian sixth (3 notes: ☭6–1–♴4, e.g. A♭–C–F♯), the French sixth (4 notes: ☭6–1–2–♴4, e.g. A♭–C–D–F♯), and the German sixth (4 notes: ☭6–1–☭3–♴4, e.g. A♭–C–E♭–F♯). All three share the same defining augmented-sixth interval and the same outward resolution to V; they differ only in the filler tones between the two outer voices.

They share the same outer-voice skeleton (☭6 and ♴4) and differ only in the inner voices. The Italian sixth is the leanest — just three notes (☭6, 1, ♴4), with no duplication, giving a spare, transparent color. The French sixth adds the supertonic (2nd degree): four notes (☭6, 1, 2, ♴4), which produces a subtly more dissonant, whole-tone-flavored sound because of the stacked perfect-fourth/whole-step relationships. The German sixth adds the lowered mediant (☭3): four notes (☭6, 1, ☭3, ♴4), making it enharmonically identical to a dominant seventh chord built on ☭6 (A♭7 in C major) — the only difference is the spelling (♴4 F♯ instead of ☭5 G♭) and the direction of resolution. The German sixth is the most common of the three in romantic and film music because its fuller, richer texture carries more emotional weight.

By expanding outward to the dominant (V). The defining augmented-sixth interval (♴4 to ☭6) resolves by semitone in both directions onto the octave of V: the ♴4 rises a chromatic semitone to become the leading tone of V, and the ☭6 falls a chromatic semitone to become the 5th of V. In C major, F♯ → G and A☭ → G. The three most common cadence patterns are: (1) +6 → V → I, the direct resolution; (2) Ger+6 → I6/4 → V7 → I, the most dramatic classical cadence, inserting the cadential 6/4 between the augmented sixth and the dominant; and (3) +6 → V7 → i in minor keys, where the augmented sixth darkens an already-dark tonality. The signature sound is the two outer voices squeezing toward V from opposite directions — once you hear it, you will recognize it in everything from Beethoven sonatas to John Williams film scores.

They are enharmonically identical but functionally different. The German sixth in C major (A♭–C–E♭–F♯) sounds exactly the same as the A♭ dominant seventh (A♭7 = A♭–C–E♭–G♭). The difference is entirely notational and behavioral: the German sixth is spelled with F♯ and resolves outward to G (V of C), while A♫7 is spelled with G♭ and resolves as a dominant — typically down a fifth to D♭ (its V/I relationship) or, in jazz, as a tritone substitute for D7 resolving to G. The tritone substitution (CHO-9748 chord-substitution lesson) and the augmented-sixth tradition are two theoretical frameworks for the same sonority. Jazz players think "tritone sub"; classical players think "German sixth." Both are correct — the spelling you choose signals which resolution you intend.