The nota cambiata

The licensed exception to the passing rule: the line steps down onto a dissonance, then leaps down a third away from it instead of resolving by step, then turns and steps up. The figure works because its fourth tone supplies the resolution the dissonance skipped, one note late — the ear hears the ornamented descent of a step, delivered as four notes. The shape is fixed: step down, leap of a third down, step up; other arrangements are ordinary errors, not cambiatas.

It earns its license by beauty: the four-note shape is more graceful than the plain step it decorates, and composers from the polyphonic golden age forward used it as a signature of the singing style. Knowing it also protects the reader of scores — a leap from a dissonance that would be an error anywhere else is correct here, provided the shape is exact.

• Resolving nowhere: the fourth tone must supply the step the dissonance skipped.
• Leaping up from the dissonance, or leaping more than a third, which is outside the licensed shape.
• Starting the figure so the dissonance falls on the strong quarter, when it belongs to the weak one.
• Inverting the figure casually — the classic cambiata descends; the mirrored form is a rarity, not a default.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, third species (the nota cambiata) — The one licensed leap from a dissonance, taught inside the quarter-note species.
• Luigi Cherubini, Cours de contrepoint et de fugue (1835), Third species (the changing note) — The Conservatoire transmission of the same fixed shape.