The compound cadence

The authentic close dignified with a dissonance on the dominant: the bass takes the dominant degree early while the upper voices still hold the tonic chord above it as a six-four, then fall by step onto the dominant’s own tones before the close. The six-four is heard as a double suspension — its fourth and sixth resolve down to the third and fifth — which is why it sits on a stronger beat than the chord it decorates, and why the bass beneath it does not move. The same staging with the fourth alone is the 4–3 form; the older thoroughbass teaching called both the compound cadence.

The standard way of dressing a cadence for emphasis: rather than letting the dominant arrive plainly, the six-four holds the ear on a suspended tonic sonority through the strong beat before releasing it, so the dominant itself seems earned rather than simply reached. Because the bass holds still beneath it, the ear reads the six-four as an ornament of the dominant rather than a chord in its own right — a reading thoroughbass players took for granted. It belongs at cadences of real weight, from an interior phrase to a movement’s final close.

• Placing the six-four on the weak beat and the plain dominant on the strong beat, which inverts the whole point of the device — the suspended dissonance belongs on the strong beat, its release on the weak one.
• Letting the bass move during the six-four itself, so the dominant seems to arrive twice instead of resolving once from beneath a held bass.
• Doubling the sixth or the fourth above the bass instead of the bass note itself, when the standard doubling belongs to the tone that is not about to resolve.
• Resolving the sixth or the fourth by leap rather than stepping down to the fifth and third.

• Fedele Fenaroli, Regole musicali per i principianti di cembalo (1775), The rules of the cadences (the compound cadence) — The middle grade of the Neapolitan closes: the dominant carrying its dissonant ornament.
• Ebenezer Prout, Harmony: Its Theory and Practice (1889), The chapter on cadences — The cadential six-four as accented double suspension, bass held, resolution downward.