Rules
- Two half notes against each whole note of the cantus firmus.
- Strong beat: always consonant.
- Weak beat: may be consonant (even reached by leap) or dissonant only as a passing tone (arrival and departure by step in the same direction).
- Parallel fifths and octaves are forbidden even between consecutive strong beats (the “accent fifths”), unless masked by a wide leap.
- You may begin with a half rest; the last note is a whole note.
- Avoid repeating the same note and arpeggiating too much.
Example in C major
Cantus firmus: C4 | E4 | …
Counterpoint above: E5 D5 | C5 …
On the first beat E5 forms a tenth (compound third) with C4; the D5 on the weak beat is a ninth, dissonant, but it is a passing tone between E and C; on the next strong beat C5 forms a sixth with E4. Consonance – passing tone – consonance.
What it teaches
- Control of rhythmic dissonance: tension is allowed where the meter makes it light.
- The first step toward bass lines and eighth-note countermelodies over slower harmonies.
Real example
In the inner voices of Bach chorales, the eighth-note passing tones between two chord tones are second species applied: every dissonance falls on a weak beat and proceeds by step.