Augmented sixth chord
A chord with an augmented sixth above the bass; named Italian, French, or German by its construction.
What it means
The augmented sixth chord is an altered formation whose hallmark is the augmented sixth above the bass. In C minor, A-flat, the lowered sixth degree, lies in the bass, and above it F-sharp, the raised fourth degree. Both notes strive apart, each by a semitone, and meet in the octave G — the root of the dominant.
Musical use
Its position is correspondingly fixed: it stands before the dominant or before the cadential six-four chord that leads to the dominant, and prepares it with a double leading-tone tension. In function theory it is interpreted as an altered form of the double dominant or as an altered subdominant; opinions on this diverge in the textbooks.
Depending on which notes stand between the two outer notes, three constructions are distinguished. The Italian one contains only the third above the bass and is three-voiced. The French one additionally adds the augmented fourth and sounds especially pungent because of the tritone it contains. The German one instead adds the fifth above the bass. The names are classificatory terms of theory and do not designate any established national origin.
The German form sounds, on tempered instruments, like a dominant seventh chord. This enharmonic ambiguity is used for modulations: the same sonority can lead, as an augmented sixth chord, into one key, and, as a D7, into a quite different one. When the German form resolves into the dominant, parallel fifths easily arise, which is why it is mostly led via the six-four chord.
The three constructions are called Italian, French, and German; all mostly lead to the dominant, often via a six-four chord. Characteristic is that the augmented sixth resolves outward to the octave. The German sixth chord sounds like a dominant seventh chord and can be reinterpreted enharmonically, which makes it a popular means of modulation.
Performance practice
Lead the two outer notes consistently apart into the octave — the effect lives on this. Intonate the upper note rather high and the lower rather low. And give the sonority weight before the dominant; it is a climax of tension, not a passing element.