Two notes in the counterpoint against one note in the cantus firmus is called second species. On the first beat of each measure, the half note in the counterpoint sounds together with the whole note in the CF. The first note is always a consonance, and the second note, which is unaccented, may be a consonance or a dissonance. A dissonance may only be approached by a step, not a leap. Thus, the only available dissonances are the passing tone or the neighboring tone.
In general, second species is characterized by occasional passing tones on the weak beat. Fux refers to Strong and Weak beats as “Thesis” and “Arsis.”