Second-Species Counterpoint

Second-species counterpoint places two notes in the counterpoint against each note of the cantus firmus, creating a 2:1 ratio. The cantus firmus is typically written in whole notes; the counterpoint moves in half notes. This introduces a crucial new concept: the distinction between strong beats (downbeats) and weak beats (upbeats). The interval on the downbeat must be consonant, just as in first species. The interval on the upbeat, however, may be dissonant — but only if the dissonant note is approached and left by stepwise motion, passing smoothly between two consonances.

The dissonant passing tone is the signature device of second species. A note on the weak beat that forms a second, fourth, or seventh with the cantus is permitted precisely because it passes stepwise between consonances. A dissonant note that is leaped to or from, or that sits on the strong beat, violates the rules. Consonant passing tones and neighbor tones are also available and carry no special restrictions beyond the parallel-fifths and parallel-octaves prohibition, which now applies to consecutive downbeats.

Voice leading across the barline — from downbeat to downbeat — must be checked for parallels. Two consecutive downbeats forming a perfect fifth or octave by similar or parallel motion are forbidden, even if the intervening weak beat breaks the visual connection. Second species builds rhythmic momentum and introduces the fundamental idea that dissonance is not prohibited outright but must be handled with care: approached correctly, placed in a metrically weak position, and resolved by step.