Cambiata and suspension — the two named licensed-dissonance exceptions

The nota cambiata (3rd species): a 4-note formula — step DOWN into a dissonance on what would otherwise be a strong sub-beat, then a LEAP DOWN a third, then step UP, then step UP again — is the one Fuxian figure where a dissonance is licensed without being left by step, because the following leap-of-a-third resolves the overall shape by phrase's end. Suspension (4th species, and 7-6/4-3/9-8 figures in free counterpoint): the dissonant note must be PREPARED as a consonance tied over from the previous strong beat, and must RESOLVE DOWN BY STEP.

A 3rd-species passage where a strong-sub-beat dissonance is left by a leap DOWN A FOURTH (not a third) — not the cambiata shape, and an unlicensed dissonance easily mistaken for the exception if the exact interval isn't checked.

Fux (Gradus ad Parnassum); Jeppesen (the cambiata's exact 4-note interval pattern is the standard modern codification of Fux's figure).

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.