Augmented Sixth Chord Voice-Leading Patterns

Core Idea

Augmented sixth chords (Italian, French, German) are chromatic chords that resolve to tonic 6/4, with the augmented sixth interval expanding outward to an octave. Each variant has specific voice-leading patterns: Italian (iv6/5), French (iv7/5#4), and German (iv6/5#3) each contain different chromatic pitches that must resolve correctly. Voices typically approach the 6/4 through stepwise motion, creating smooth resolution of the tritone interval.

Explainer

From your prerequisite on augmented sixth chords, you know the three varieties — Italian (It+6), French (Fr+6), and German (Ger+6) — and their defining feature: the augmented sixth interval between the lowered sixth scale degree (b6) in the bass and the raised fourth scale degree (#4) in an upper voice, which resolves outward by contrary half-step motion to an octave on the dominant. From voice-leading principles and seventh-chord resolution, you understand how dissonant intervals create resolution obligations. This topic focuses on the specific voice-leading patterns each variety demands and why the outward resolution of the augmented sixth is one of the most powerful pre-dominant gestures in tonal harmony.

The defining voice-leading gesture is the same for all three varieties: b6 descends by half step to 5, and #4 ascends by half step to 5. Both chromatic notes move by the smallest possible interval, in opposite directions, converging on the same pitch class — the dominant scale degree. In C major or C minor, Ab moves down to G while F# moves up to G, producing a perfect octave on G (scale degree 5). This contrary half-step motion from both sides creates an effect of intense chromatic compression releasing into consonant arrival — the augmented sixth literally squeezes inward from both directions. No other pre-dominant harmony has this dual half-step approach to the dominant; diatonic pre-dominants (IV, ii) approach V by whole step or larger intervals, making the augmented sixth uniquely urgent.

The three varieties differ in how the remaining voices behave. The Italian sixth has only three pitches (b6, 1, #4), so in four-voice writing, one pitch must be doubled — typically scale degree 1. Voice leading is straightforward: the doubled 1 stays in place or moves to a nearby tone of the resolution chord. The French sixth adds scale degree 2, which must resolve down by step to the leading tone (7) or up to 3 in the resolution chord, depending on the specific voicing. The French sixth's extra dissonance — scale degree 2 creates a tritone with b6 — gives it a sharper, more pungent sound. The German sixth adds b3, which descends by step. Because the German sixth is enharmonically equivalent to a dominant seventh chord, it carries a voice-leading complication: resolving directly to V would create parallel fifths between b3 (moving down to 2) and b6 (moving down to 5). Composers typically avoid this by resolving the German sixth to a cadential 6/4 (tonic in second inversion) first, which then resolves to V — the 6/4 acts as a voice-leading buffer.

The resolution to tonic 6/4 followed by V is the standard cadential context for augmented sixth chords. The full voice-leading pattern in C major looks like: Ger+6 (Ab-C-Eb-F#) resolves to the cadential 6/4 (G-C-E-G), which then resolves to V (G-B-D-G), which finally resolves to I. The augmented sixth chord intensifies the approach to the dominant far beyond what a diatonic ii or IV chord can achieve — the chromatic half-step compression from both sides makes the arrival on V feel inevitable and powerful. This is why augmented sixth chords appear so frequently at climactic cadences in Classical and Romantic music: they provide the maximum possible pre-dominant tension, delivered through the most directed possible voice leading.