German sixth chord
A German sixth chord is a tetrad comprising a root, major third, fifth, and augmented sixth.
In meantone (including 12edo), on which traditional tonal harmony is built, the augmented sixth is treated as a rare and special dissonance. It is approximately the size of a harmonic seventh (7/4), so the chord approximates the harmonic seventh chord:
- 1 – 5/4 – 3/2 – 7/4 with steps 5/4, 6/5, 7/6.
In 5-limit just intonation, this augmented sixth is likely to be 225/128 – the Neapolitan augmented sixth, so if we stick to the 5-limit, one possible tuning is given as 128:160:192:225.