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The cadential 6/4 chord (I6/4 in the context of V) extends the dominant function before resolving to V, creating a characteristic gesture in Classical music. Voice-leading requirements are strict: both the sixth and fourth above the bass are typically prepared as chord tones from the previous chord and resolve by step. The 6/4 functions as an embellishment of dominant harmony, making careful voice-leading analysis critical to understanding its role in phrase structure.
The cadential six-four is one of the most distinctive sonorities in tonal music, and it is also one of the most misunderstood. Its Roman numeral label, I6/4, appears to identify it as a tonic chord in second inversion. This labeling is technically accurate but functionally misleading. The cadential 6/4 does not behave like a tonic chord — it does not provide rest or arrival. Instead, it functions as an intensification of dominant harmony, and understanding this distinction is the key to both analyzing and composing with it correctly.
Here is why. Your prerequisite study of chord inversions showed you that a 6/4 chord has scale degree 5 in the bass. When I6/4 appears over a bass that is already moving toward (or staying on) scale degree 5, those upper voices — scale degree 1 and scale degree 3 — are actually suspended dissonances above the dominant bass. The "tonic" notes in the upper voices create tension against the dominant bass, and that tension resolves when those notes step down: 1 resolves to 7, and 3 resolves to 2 — arriving at V in root position (with 5 in the bass and the chord tones 7 and 2 above it). What sounded like a tonic chord turns out to be a two-note appoggiatura sitting above a dominant bass. The "I" in "I6/4" is almost a fiction of notation.
The voice-leading rules become obvious once you see the cadential 6/4 this way. Both the sixth (scale degree 1 in the soprano or tenor) and the fourth (scale degree 3) above the bass must resolve downward by step — just like suspensions or appoggiaturas over any dominant bass. They arrived as chord tones from the preceding harmony (preparation), they sit as dissonances against the dominant bass (suspension point), and they step down to the seventh and fifth of V (resolution). This is the paradigm of suspension resolution you already know, applied simultaneously to two voices over a stationary bass.
In Classical phrase structure, the cadential 6/4 almost always appears at the end of a phrase, just before the final V–I. It is a moment of prolonged anticipation — the harmonic equivalent of a held breath before the exhale of the authentic cadence. Mozart and Haydn use it constantly in piano sonatas and string quartets: the texture thins, the bass anchors on scale degree 5, and the upper voices hover on those unstable 1 and 3 scale degrees for a beat or two before resolving to V and then I. Training yourself to spot this gesture — that distinctive moment of suspended tonic notes over a dominant bass — will make Classical phrase endings far more transparent in your analysis and more controlled in your compositions.
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