Three part counterpoint allows more freedom of vertical movement and harmonic interest, but with more freedom comes more happy fun rules. So put your helmet on.

  1. You should never write one part against the given cantus and then go back and try to write another part - you'll trap yourself with the two parts you already wrote. It's better to work measure by measure, so you can understand how each note interacts with (or acts as) the harmony and then with each other.

  2. The tonic triad must be your harmony in the first and last measures. Some teachers will tell you it can be an incomplete triad (complete being root, third and fifth), but I think for the sake of the counterpoint it should be the complete form of the triad.

  3. The measure before the last measure should be a complete harmony. Again, it can, in extreme situations, be an incomplete triad but this is highly undesirable.

  4. Logically, any leading tones and resolving tonics must be in the same part.

  5. While the lowest voice of your exercise (cantus or not) must begin and end on the tonic, the upper two parts don't have to.

  6. The top two parts should never have more than a tenth between them, and any separation of an octave or more should be momentary. This doesn't apply to the bass voice if you are composing that against a higher cantus. In that case any appropriate interval is allowed between the bottom voice and the voices above. You can also cross the top two voices briefly but never drop an upper voice below the bass.

  7. Fourths are allowed amongst the upper voices, and so long as your triad is complete you can even have tritones between the upper voices. But only every so often... remember, tritones summon the Devil!

  8. You may write up to 2 consecutive 3rds, 4ths, or 6ths, and that's it.

  9. Try to avoid unisons among the upper voices, but if it keeps you out of serious trouble you can dip in for a short short duration.

  10. Hidden octaves and fifths when brought about by direct movement of an outer and inner part are permitted when:
    • they are moving within the same harmony
    • they form a part of the resolution in the last measure
    • one part is moving stepwise.

  11. A good general guideline is: Try to not arrive at a fifth or octave by skips in direct movement in any two parts. If you can manage that, you should be okay.

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