Six-five chord
First inversion of the seventh chord.
What it means
The six-five chord is the first inversion of a seventh chord: the third lies in the bass, and above it stand the fifth, the seventh, and the root. The name comes from figured-bass notation, which counts the intervals above the bass — the characteristic ones are the fifth and the sixth, designated 6/5.
Musical use
Most often it appears as the inversion of the dominant seventh chord. Then the leading tone lies in the bass, leading upward to the root of the tonic; in terms of function theory the sonority is designated as D7 with the third in the bass. The resolution proceeds with the bass moving a semitone step upward into the root position of the tonic — a smooth, forceful movement that makes the close less final than the root position but distinctly goal-directed.
The seventh lies in an upper voice and is resolved downward as usual. Because the leading tone stands in the bass, it must not be doubled in an upper voice; otherwise parallel octaves would arise at the resolution.
A second common use concerns the subdominant: the subdominant with an added sixth yields, above the root, a sonority identical with the six-five chord of a seventh chord on the second degree. This ambiguity — as sixte ajoutée or as an inversion — is weighted differently in theory; in function theory the interpretation as a subdominant with an added sixth predominates.
With the dominant seventh chord in first inversion, the leading tone lies in the bass, leading upward to the tonic. In figured-bass notation it appears as 6/5. In function theory one writes D7 with a 3 under the symbol. A particular role is played by the six-five chord of the subdominant with an added sixth, which Rameau described as the sixte ajoutée and which stands in the cadence before the dominant.
Performance practice
Lead the bass note in the six-five chord upward — it is usually the leading tone and wants to go to the tonic. Resolve the seventh downward. And determine, when in doubt, from the context whether the sonority works as dominant or subdominant; this decides the shaping.