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What may each note do against the cantus firmus?
The graded discipline of species counterpoint: five species against a fixed voice, each admitting one new rhythmic freedom and one new class of dissonance. The species are not repertoire — they are the training frames in which consonance, passing motion, neighbor motion, syncopation and florid synthesis are first practiced.
The fixed voice every species exercise is built against is itself composed to rules: it moves in equal whole notes, begins and ends on the final of its mode, rises to a single climax it touches once, proceeds mostly by step with any leap modest and answered, stays within about a tenth, and approaches its last note by step.
Note against note, consonance against consonance: both voices move in whole notes, every interval is consonant, the exercise begins and ends on a perfect consonance, and imperfect consonances — thirds, sixths, tenths — carry the interior because they keep the voices distinct.
Two notes against one: the counterpoint moves in halves against the cantus firmus’s wholes, entering after a half rest.
Four notes against one.
The syncope: the counterpoint enters on the weak half and ties every note across the bar, so each bar opens with the held tone against the new bass note.
Florid counterpoint: all the species at once, in the mixture a real melody would use — halves, quarters, ties and their suspensions, an occasional pair of eighths — each figure still governed by its own species’ rule.
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.
Both neighbors taken at once: the line steps off a held tone to one neighbor, leaps a third across to the opposite neighbor, and returns to the tone it decorated.