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Here we reach the heart of the matter. Hundreds of volumes have been written on the topic of how to successfully combine two or more melodies - that is what counterpoint is all about, and the Counterpointer software understands about 170 possible rules and exceptions that help it to evaluate your writing. But we can get started with just a few very basic principles, and work up from there:
1. Melodies in all voices should follow the principles for single melodies described earlier.
2. A voice should not leap to a dissonance, outside of certain exceptions to be described later.
3. Dissonances formed between voices should be resolved by stepwise movement to a consonant interval.
4. Voices should try to move in opposite directions as much as possible and generally give an impression of independence.
5. If two voices form a perfect interval they should not move directly to another perfect interval, especially in the same direction.
If you followed only the above 5 principles you'd probably do OK most of the time. But if you make a more complete study you'll gain skill in realizing those principles and you'll learn many techniques that will add variety and interest to your writing. The time-honored way of proceeding is to begin with Species Counterpoint.
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