What is an Augmented Sixth Chord?

An augmented sixth chord is a type of chromatic chord used in tonal harmony that contains the interval of an augmented sixth between two of its notes. This interval strongly resolves outward to the dominant note of the key, creating a powerful sense of tension and resolution.

The chord usually includes the lowered sixth scale degree (♭6) and the raised fourth scale degree (#4) of the key. These two notes form the augmented sixth interval and move outward by step to the fifth scale degree, which is the root of the dominant chord. Because of this strong resolution, augmented sixth chords are often used just before the dominant in a progression.

There are three common types of augmented sixth chords in classical harmony: the Italian sixth, French sixth, and German sixth. Each version contains the same core augmented sixth interval but includes different additional notes. These chords became especially prominent in Classical and Romantic music, where composers used them to add dramatic chromatic tension before resolving to the dominant chord.