Hidden (direct) fifths and octaves in outer voices

Two voices moving by SIMILAR motion INTO a perfect 5th or octave — even though the prior interval was not itself perfect — is a 'hidden' or 'direct' fifth/octave. Species pedagogy (Fux via Jeppesen) forbids this universally between any two voices. Common-practice four-part-harmony pedagogy (Kostka & Payne; Aldwell & Schachter) narrows the prohibition to the OUTER voices (soprano/bass) specifically, and only when the UPPER voice (soprano) LEAPS rather than moves by step into the perfect interval — if soprano moves by step, the hidden interval is permitted. RECOMMENDATION for Gradus: apply the strict universal rule in species-counterpoint content and the narrower outer-voice-plus-soprano-step-exception rule in chorale/four-part-harmony content, matching what each unit's own textbook teaches.

Soprano leaps C4→A4 (a 6th) while Bass moves by similar motion, landing on a perfect 5th or octave at the same onset, in four-part-harmony content — a hidden-fifth/octave violation since soprano moved by leap.

Species counterpoint (Fux via Jeppesen) forbids universally.

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