Species melodic shape — single climax, no repeated notes, cadence approach

A well-formed species counterpoint line has exactly ONE climax (highest note), approached by step, not leap. Repeated notes (same pitch on consecutive onsets within the counterpoint line) are forbidden in 1st-3rd species, with a narrow allowance in modern editions for later species. The final cadence: the penultimate note moves to the final by STEP, from a major 6th (9 semitones) expanding to an octave (12 semitones), or a minor 3rd (3 semitones) contracting to a unison (0 semitones), in two-voice counterpoint — the two standard cadential approach shapes cited by Fux/Jeppesen.

A counterpoint line with two notes tied for the highest pitch, one reached by an upward leap of a 4th — violates both the single-climax and approached-by-step requirements simultaneously.

Fux (Gradus ad Parnassum); Jeppesen (Counterpoint) — 'single climax approached by step' and 'no repeated notes' are both explicit pedagogical requirements, with Jeppesen noting modern flexibility on the repeated-note rule that Fux's original treatise did not extend.

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