First species — note against note

Note against note, consonance against consonance: both voices move in whole notes, every interval is consonant, the exercise begins and ends on a perfect consonance, and imperfect consonances — thirds, sixths, tenths — carry the interior because they keep the voices distinct. Perfect intervals are approached in contrary or oblique motion, never similar; more than three parallel thirds or sixths in a row surrenders the voices’ independence. First species is harmony reduced to its skeleton: two lines agreeing without either one dissolving into the other.

It is where two-voice judgment is formed: with rhythm removed, nothing distracts from the quality of each vertical interval and the motion between them. The species survives far beyond the classroom — note-against-note reduction is still the fastest way to check any passage’s voice-leading skeleton, and homophonic writing is first species with rhythm restored.

• Approaching a fifth or octave in similar motion, the direct motion the species exists to train out.
• Stringing more than three parallel thirds or sixths, so the two voices fuse into one doubled line.
• Using the unison anywhere but the first and last bars, where it swallows a voice.
• Overusing perfect consonances in the interior, which empties the texture the imperfect intervals exist to fill.
• Crossing the voices to rescue an interval, trading a local repair for a lost registral identity.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, first species (note against note) — The founding exercise: whole note against whole note, consonance only.
• Luigi Cherubini, Cours de contrepoint et de fugue (1835), First species, two-voice counterpoint — The Conservatoire codification of the same rules, with the parallel-thirds limit stated.