Physics: Bach's 4-part chorale style (SATB): soprano/alto/tenor/bass. Rules: avoid parallel 5ths/8ths (produces hollow, empty sound), prefer contrary motion, stepwise movement preferred over leaps, leading tone (7th scale degree) must resolve up. Fux (1725) Gradus ad Parnassum codified 5 species of counterpoint. Heinrich Schenker: all tonal music is an elaboration of the fundamental I–V–I Ursatz. Voice leading efficiency = minimize total semitone displacement across all voices.