French sixth
An [augmented sixth chord](/term/augmented%20sixth%20chord) containing four notes, distinguished by an augmented fourth above the bass
In Depth
The French sixth (Fr+6) adds an augmented fourth (or tritone) above the bass to the basic Italian sixth structure. In C minor, it would be A♭-C-D-F♯, containing two tritones (A♭-D and C-F♯). This double-tritone content gives the French sixth a particularly tense, unstable quality. The chord is whole-tone in character — all four notes belong to the same whole-tone scale — which connects it to the impressionist harmony of Debussy and Ravel.
The French sixth's whole-tone character was exploited by Debussy, who used it as a gateway between tonal and whole-tone passages.