Second species — two against one

Two notes against one: the counterpoint moves in halves against the cantus firmus’s wholes, entering after a half rest. The strong half of every bar must be consonant; the weak half may be consonant by step or leap, or dissonant only as a passing tone — approached and left by step in one direction. The species adds exactly one liberty to the first: dissonance in motion, never at rest.

The first taste of rhythm against rhythm: the weak half lets the line walk between the pillars first species set. Its passing tone remains the most used dissonance in all of music — every style that came after kept it — so drilling its conditions here, weak beat, stepwise, one direction, pays off everywhere from chorale to sonata.

• Placing a dissonance on the strong half, which second species has no license for.
• Treating the weak half’s freedom as freedom to leap into dissonance — the passing tone is entered and left by step, in the same direction.
• Repeating the strong note on the weak half, which halts the motion the species is meant to practice.
• Landing successive strong beats on parallel fifths or octaves, which the intervening weak note does not excuse.
• Beginning on the downbeat with an imperfect consonance instead of entering after the half rest on a perfect one.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, second species (the passing dissonance) — The species that first admits dissonance — in passing, on the weak half.
• Luigi Cherubini, Cours de contrepoint et de fugue (1835), Second species, two-voice counterpoint — The same discipline in the Conservatoire course, including the strong-beat rule.