The 9–8 suspension

A tone prepared as a consonance is held while the bass moves so that it strikes a ninth above the new bass on the strong beat, then falls one step to the octave. Because its resolution doubles the bass, the 9–8 is the heaviest of the upper-voice suspensions: at the strike the texture should give the ninth the bass alone to resolve against — root, third and fifth below it, never the octave already sounded in another upper voice.

Because its resolution doubles the bass, the 9–8 suits the weightiest arrivals — a final tonic, a structural downbeat — where a plain V–I would leave the root under-reinforced. It asks more of the texture than the other suspensions: the tone below the ninth must be free of the octave elsewhere, which is why it sits most naturally in four or more voices. Set over a dominant resolving home, it adds a full voice’s weight to a cadence that hardly needs the help — saved for the close that must sound final beyond argument.

• Doubling the octave — the tone of resolution — in another upper voice while the ninth is still suspended, so there is no clear space left for the suspension to resolve into.
• Attacking the ninth without preparing it as a consonance in the previous chord, which turns a controlled suspension into a bare dissonant leap.
• Resolving the ninth upward or by skip rather than falling one step to the octave.
• Reaching for a 9–8 in a texture too thin to spare a voice for it, forcing an incomplete chord at the moment the harmony most needs to sound full.
• Voicing the suspension so close to the bass that the ninth collapses into the harsher clash of a plain second, losing the spacious quality a compound interval was chosen to give it.

• Jean-Philippe Rameau, Traité de l’harmonie réduite à ses principes naturels (1722), The chapters on supposition (the ninth) — Rameau reads the ninth as a tone set below a seventh chord — the suspension explained through the fundamental bass.
• Ebenezer Prout, Harmony: Its Theory and Practice (1889), The chapter on suspensions — The classroom form: preparation, strike over the new root, stepwise fall to the octave.