Strong-beat consonance requirement, per species

1st species (note-against-note): every simultaneity must be a consonance (unison, 3rd, 5th, 6th, octave; the perfect 4th is dissonant against a bass-position CF, consonant between upper voices). 2nd species (2:1): the DOWNBEAT of each pair must be consonant; the offbeat may be a passing/neighboring dissonance. 3rd species (4:1): beat 1 of each group of 4 must be consonant, except the cambiata figure; beats 2-4 may carry passing/neighbor dissonances. 4th species (syncopated): the note tied into the downbeat may be dissonant if consonant in its previous weak-beat position — an explicit exception to the downbeat-consonance rule.

A 1st-species exercise with any simultaneity forming a 2nd, unresolved 4th against the bass, 7th, or augmented/diminished interval. A 3rd-species exercise with a non-cambiata dissonance on beat 1 of a group of 4.

Fux (Gradus ad Parnassum); Jeppesen's Counterpoint (modern standard codification of beat-role specifics).

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