The principle
They are chromatic predominants that contain the interval of an augmented sixth between the lowered 6th degree (in the bass) and the raised 4th degree. The two notes spread apart by semitone to the octave on the dominant.
In A minor (dominant E)
- Italian (It+6): F A D#. F falls to E, D# rises to E.
- French (Fr+6): F A B D#. It contains two major thirds stacked at a tritone: an “impressionist” sound.
- German (Ger+6): F A C D#. It sounds like F7 (D# = Eb).
In C major (dominant G)
- It+6: Ab C F#
- Fr+6: Ab C D F#
- Ger+6: Ab C Eb F#
All go to G or to C/G (cadential 6/4) – G7 – C. The German sixth preferably goes to C/G: if it went directly to G, Ab–Eb → G–D would form parallel fifths (the so-called “Mozart fifths,” tolerated in certain contexts).
Link with jazz
The German sixth in C (Ab C Eb F#) is enharmonic with Ab7: it is the same chord as the tritone substitution of D7 (V/V) going to G. Same sonority, different names: in one context it is called an augmented sixth, in the other bVI7 or subV/V.
Compositional use
- Before the return of a theme or a drop: it magnifies the dominant.
- In choirs: the two outer voices spreading apart by semitone are easy to sing and very effective.
Real example
The “Pink Panther” theme (Henry Mancini, 1963) and the Overture to Rossini's “William Tell” are cited as examples of the German sixth; the first movement of Beethoven's Fifth contains an Italian sixth.