Why Does a Cadential Six-Four Not Sound Like a Resting Tonic?

A cadential six-four is often labelled I⁶⁄₄: its three pitches do belong to the tonic triad, yet it rarely feels as if the music has arrived home. The reason lies in the bass. Scale degree five sits underneath, while the tonic and third appear above it as a fourth and a sixth, normally resolving down to the third and fifth of the dominant. It is more useful to hear the sonority as a dominant expanded by two notes awaiting resolution than as a tonic at rest.

The cadential example in the first movement of Mozart's Piano Sonata in A major, K.331, makes the process clear. The bass establishes the dominant as the centre of gravity; movement in the upper voices concentrates the tension into V, which then resolves to I. The pitch names in adjacent chords matter, but what we hear is also determined by the bass, each voice's destination and the chord's position in the cadence. Playing I⁶⁄₄ as a self-contained tonic can produce a premature arrival; passing over it entirely removes the cadence's expansion.

At the piano, the bass should retain its direction while the fourth and sixth above it fall into the dominant as unresolved voices. The true release can then be reserved for the final tonic. The chord is a useful reminder that harmonic function is not merely a label for a collection of notes: it is the direction those sounds take through time.

https://viva.pressbooks.pub/openmusictheory/chapter/cadential-64/
https://musictheory.pugetsound.edu/mt21c/TheCadentialSixFourChord.html
https://imslp.org/wiki/Piano_Sonata_No.11_in_A_major%2C_K.331/300i_%28Mozart%2C_Wolfgang_Amadeus%29