The stream of sixth chords

A bass descending by step may carry a chord of the sixth on every tone: because the 6/3 contains no perfect fifth against the bass, the whole texture can move in parallel — thirds and sixths all the way down — without breaking a rule. It is the one standard texture in which parallel motion is the point: the voices give up independence deliberately, and the stream reads as one harmonized line rather than a progression of chords.

Use it to harmonize a stepwise descent without weight: the stream carries a line downhill quickly and lightly, which is why it serves as the middle of phrases rather than their ends — it moves, but nothing in it cadences. It is equally a color: from the fauxbourdon of the fifteenth century to orchestral passages in parallel sixth chords, the texture reads as one voice wearing harmony, and composers reach for it exactly when that fused sound is wanted.

• Slipping a root-position triad into the stream, whose fifth immediately makes parallels with its neighbors.
• Ending on a sixth chord, which cannot close — the stream must hand off to a root-position cadence.
• Doubling the bass of one of the sixth chords in an outer voice mid-stream, breaking the parallel surface.
• Running the stream so long that the key blurs, since nothing in it asserts a tonic.

• Fedele Fenaroli, Regole musicali per i principianti di cembalo (1775), The rules for the movements of the bass (the bass falling by step) — The falling-step movimento carried on chords of the sixth.
• Ebenezer Prout, Harmony: Its Theory and Practice (1889), The treatment of sequences — The parallel 6/3 stream as the standard harmonization of a falling scale.