The rising 5–6

A bass that wants to climb by step cannot carry parallel triads — the fifths would march in parallel. The 5–6 exchange is the tradition’s repair: over each bass tone the fifth becomes a sixth before the bass moves, so every fifth is left by oblique motion and the sixth turns into the next tone’s fifth. The exchange converts a forbidden parallel ascent into a self-renewing sequence, and it will climb as far as the line needs.

Reach for it whenever a bass ascends by step for more than two tones — scale-wise basses in partimenti, rising transitions, any climb toward a dominant. It is also the analytical key to countless passages that look like parallel chords and are not: the little 6 between the fifths is doing all the legal work, and hearing the exchange turns a puzzle of chords into one motion.

• Omitting the exchange and marching root-position triads up the scale in parallel fifths.
• Moving the bass before the sixth has sounded, which restores the very parallels the exchange exists to break.
• Doubling the tone of the sixth at the moment of exchange so the following fifth arrives doubled and parallel.
• Continuing the climb past the goal degree because the sequence renews itself so easily.

• Fedele Fenaroli, Regole musicali per i principianti di cembalo (1775), The rules for the movements of the bass (the bass rising by step) — The Neapolitan assignment: 5 then 6 over each rising bass tone.
• Ebenezer Prout, Harmony: Its Theory and Practice (1889), The treatment of sequences — The classroom treatment of the rising sequence and its parallel-breaking sixth.