Sixte ajoutée

A triad with a sixth

The triad F–A–C is the subdominant of C major. If you add a sixth above the root, here the D, F–A–C–D arises. A seventh does not belong to it. That is the sixte ajoutée. The term was coined by Jean-Philippe Rameau in his 18th-century theory.

In the glossary: Six-five chord · Subdominant · Functional theory

Listening

The second chord is the subdominant with the added D. It sounds warmer than a plain F major chord and leads to the dominant or to the tonic. The sixth strengthens the subdominant character and prepares the dominant. In functional theory it is written S⁶₅.

Two readings of the same chord

This chord can also be read differently: as the six-five chord of the seventh chord on the second degree. Rameau himself spoke of the “double emploi”, the double use. Which function is meant is shown only by the treatment of the dissonance. The reading is not undisputed: in functional theory the subdominant with added sixth prevails, in degree theory the inversion.

Rameau treated the sixth as the characteristic dissonance of the subdominant, since it forms a second with the fifth. Unlike the seventh, which is led downwards, it resolves by step upwards while the root falls a fourth.

The tonic chord with a sixth

A second use is quite different: the sixth on the tonic triad as a final chord. Here it is not resolved but stays as a colour, and the close sounds soft and open instead of concluded. In jazz and the popular music of the 20th century this chord is widespread and belongs to the usual chord symbols: C6 means C–E–G–A. In the music of Impressionism such chords also appear as tone colour.