Consonant and dissonant
Unisons, 3rds, 5ths, 6ths and octaves are consonant, and so are the same intervals an octave higher (a 10th is a 3rd). 2nds, 4ths, 7ths and the tritone are dissonant. In two voices the 4th counts as a dissonance.
Species counterpoint rules
The rules that hold in every species of counterpoint: consonance and dissonance, parallel and direct fifths and octaves, leaps, the climax and the cadence, with examples you can hear.
Two lines, one given
You get a fixed melody, the cantus firmus: whole notes, mostly by step, starting and ending on the final of its mode. You write a second line above or below it. The rules on this page apply in every species; each species then adds its own.
Intervals
Begin on a perfect consonance
Above the cantus: unison, 5th or octave. Below it: unison or octave only, so the mode stays clear.
End on a unison or octave
And use the unison nowhere else: in the middle the two voices merge into one.
The cadence
Close with a major 6th opening to an octave (line above) or a minor 3rd closing to a unison (line below). In every mode except Phrygian this means raising the leading tone, here C to Cโฏ.
Motion between the voices
No parallel 5ths or octaves
Two perfect intervals of the same size in a row, with both voices moving, make the voices lose their independence. Parallel 3rds and 6ths are fine.
No direct 5ths or octaves
Approach a perfect interval by contrary or oblique motion, never with both voices moving the same way.
Keep the voices apart and in order
No crossing: the upper line stays above. Keep them within a 10th of each other where you can.
Prefer contrary motion
More than three 3rds or 6ths in a row, or too many bare 5ths and octaves, make the two lines sound like one.
Shape of the line
Move mostly by step
Allowed leaps: 3rds, perfect 4ths and 5ths, the rising minor 6th and the octave. Avoid the tritone, 7ths and augmented intervals: they are hard to sing.
Recover from leaps
After a large leap, step back in the opposite direction. Avoid two leaps in the same direction.
One climax
Give the line one highest note, away from the cantus's high point, and keep its range singable: within a 10th.
Don't outline a tritone
When the line runs in one direction from F up to B (or down from B to F), the ear hears the tritone between its ends, even though every move is a step. Turn back before you reach it, or carry on past it.
The modes
Six church modes
Dorian, Phrygian, Lydian, Mixolydian, Aeolian and Ionian, on several finals each. Stay inside the mode. The only chromatic note is the raised leading tone at the cadence; Phrygian never raises it.
Practise it now
Write species counterpoint rules exercises in your browser. Every rule is checked as you write, and you can hear your lines.