Six-four chord
Second inversion: the fifth lies in the bass. Counts as needing clarification and often stands before the cadence.
What it means
The six-four chord is the second inversion of a triad: the fifth lies in the bass, above it stand the root and the third at the distance of a fourth and a sixth from the bass — hence the name. In figured-bass notation it is designated 6/4.
Musical use
Because of the fourth above the bass, which is treated as dissonant in strict writing, the six-four chord counts as needing clarification: it does not stand freely but demands a particular environment. Three uses are common.
The cadential six-four chord is the most important. It appears before the dominant on an accented beat, and its upper voices resolve stepwise downward — the sixth into the fifth, the fourth into the third — while the bass stays put. In terms of function theory it is therefore interpreted not as the tonic in inversion but as the dominant with a double suspension, and designated accordingly. In concerto movements, a fermata often stands at this point, after which the soloist plays the cadenza.
The passing six-four chord arises when the bass moves stepwise between two positions of the same sonority and the six-four chord lies between them on a light beat. The neighbouring or pendulum six-four chord, finally, arises over a sustained bass when the upper voices briefly move away and return.
In all three cases the bass is usually doubled in four-part writing.
Several kinds are distinguished: the cadential six-four chord, which stands as a suspension before the dominant, the passing six-four chord between two positions of the same chord, and the neighbouring six-four chord over a sustained bass. In function theory the cadential six-four chord is understood as a double suspension of the dominant, notated D with 6/4 and 5/3. In the Classical solo concerto it often stands immediately before the solo cadenza, which extends it, before the orchestra continues with the dominant.
Performance practice
Give the cadential six-four chord weight and let it resolve into the dominant — it is tension, not a point of rest. Lead the upper voices stepwise downward. And never use it incidentally in your own harmonizing: it demands a continuation, and without one the setting sounds unfinished.