German sixth
An augmented sixth chord containing four notes, distinguished by a perfect fifth above the bass
In Depth
The German sixth (Ger+6) adds a perfect fifth above the bass to the Italian sixth structure. In C minor, it would be A♭-C-E♭-F♯. This spelling is enharmonically equivalent to a dominant seventh chord (A♭-C-E♭-G♭ = A♭7), which creates a useful modulatory ambiguity. The German sixth resolves to a cadential 6/4 chord to avoid parallel fifths. Its richly consonant quality made it a favourite of Romantic composers, particularly Wagner and Bruckner.
The German sixth's enharmonic equivalence with a dominant seventh chord is the basis of the enharmonic modulation technique that Wagner used to create seamless key changes.