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Augmented sixth chords are chromatic pre-dominant harmonies built on the flattened sixth scale degree, characterized by an augmented sixth interval between the bass (b6) and an upper voice (raised 4, enharmonically equivalent to the leading tone of the dominant). This interval expands outward by half step to the octave on scale degree 5, creating a powerful pull toward the dominant. The three standard varieties differ by their inner voices: the Italian (It+6) contains only b6, 1, and #4; the French (Fr+6) adds scale degree 2; and the German (Ger+6) adds b3. The German augmented sixth is enharmonically equivalent to a dominant seventh chord and can be exploited for enharmonic modulation.
Learn to spell each variety from the bass note up: 'It has 3 notes, Fr adds 2, Ger adds b3.' Play each in C minor to hear the characteristic augmented sixth interval expanding to the octave on G. Find examples in Schubert lieder and Beethoven sonatas, where these chords are especially frequent.
You understand interval quality from your prerequisite work, so think about what an augmented sixth actually sounds like: it's one semitone wider than a major sixth. An augmented sixth from Ab to F# spans 10 semitones. That stretched interval creates an acute sense of outward tension — the two pitches want to move *away* from each other by half step. This is the defining acoustic property of augmented sixth chords: the Ab resolves *down* to G (scale degree 5), and the F# resolves *up* to G. Both outer voices converge on the same pitch from opposite directions, arriving on an octave. That arrival note — G — is scale degree 5, the dominant. This is why augmented sixth chords are pre-dominant harmonies: their resolution is to the dominant, not the tonic.
To spell any augmented sixth chord, start from the bass note, which is always the flattened sixth scale degree (b6). In C major or C minor, that's Ab. The characteristic upper voice is the raised fourth scale degree (#4), which in C is F#. These two notes form the augmented sixth interval (Ab–F#) that gives the chord family its name. The three standard varieties differ by what fills the middle: the Italian augmented sixth (It+6) contains only those two notes plus scale degree 1 — the thinnest, most austere version. The French augmented sixth (Fr+6) adds scale degree 2, giving it a slightly more dissonant, colorful sound. The German augmented sixth (Ger+6) adds the flattened third (b3), producing a chord that, if you respell the F# as Gb, is enharmonically identical to a dominant seventh chord.
That enharmonic equivalence — Ger+6 in C = Ab7 spelled enharmonically — is both a notational trap and a compositional opportunity. As a trap: the chords look the same on paper but function completely differently. Ab7 (as a dominant seventh) resolves to Db major (its tonic). The German augmented sixth in C resolves to G (the dominant of C). The spelling tells you the function. As an opportunity: composers exploit this enharmonic equivalence to pivot between keys. A Ger+6 in one key is reinterpreted as a dominant seventh chord in another key, enabling smooth modulation. This technique appears in Schubert and Beethoven, where a chord suddenly shifts its tonal function under the enharmonic reinterpretation.
In Roman numeral analysis, you've learned to label chords by their scale degree. Augmented sixth chords don't fit cleanly into this system — they're best labeled by variety (It+6, Fr+6, Ger+6) and understood functionally as pre-dominant chromaticisms. When analyzing, look for the b6 in the bass and the #4 in an upper voice. In minor keys, b6 is already in the key signature; in major keys, it requires a flat accidental. The #4 always requires an accidental, since it's raised above the diatonic scale. Those two accidentals together — one flatted bass note, one sharped upper voice — are your signal to label what follows as an augmented sixth chord.
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