What is a French Augmented Sixth?

A French augmented sixth chord (often called the French sixth or Fr+6) is a type of chromatic chord used in tonal harmony that contains the distinctive interval of an augmented sixth. Like other augmented sixth chords, it is typically used to create strong tension that resolves to the dominant chord.

The French augmented sixth chord usually contains four notes: the lowered sixth scale degree (♭6), the tonic (1), the second scale degree (2), and the raised fourth scale degree (#4). The crucial interval is between the ♭6 and #4, which forms the augmented sixth. These two notes resolve outward by step to the fifth scale degree (5), the root of the dominant chord.

For example, in the key of C major, a French augmented sixth chord would contain the notes A♭ (♭6), C (1), D (2), and F♯ (#4). The A♭ and F♯ form the augmented sixth interval and move outward to G, creating a strong resolution to the dominant. The French sixth has a distinctive sound because the added second scale degree creates a slightly brighter and more open harmony compared to other augmented sixth chords.