It will help to look first at some basic principles that can guide the creation of a single melody. With those in mind we can move on to a general outline of the principles involved in combining melodies.
There of course can be no strict rules that would guide the creation of all melodies - someone is always going to break them - but some principles are surprisingly reliable and even universal in their simplest form.
So, keeping in mind that these principles are subject to violation, let us begin:
Our study is primarily of vocal melody. Similar principles apply to instrumental melody, but an instrumental style is harder to define precisely. Instruments are able to leap, for example, with far more agility and speed than a singer, and so can perform melodies that wouldn't work at all when sung. Still, however, it will be good practice to imagine that you are writing for a singer, even if you are not. Famous melodies are often ones we can sing, even if written for the violin.
Therefore the first principle will always be this: can you sing your melody? If you cannot, try to make it easier to sing. Here are some ways to accomplish that:
1. Make the melodic motion mostly stepwise, with some small leaps (a leap being any interval larger than a whole step).
2. Keep the melody within a limited range, generally within a single octave or perhaps an octave and a half at the most.
3. Avoid augmented or diminished leaps.
4. Compensate any large leaps by moving in the opposite direction immediately afterward.
5. If you make two successive leaps of any size, see that they are part of the same triadic chord.
With regard to rhythm:
6. Keep the rhythmic values large enough that a singer could easily perform them. Avoid very small note values (32nd notes and less) unless the tempo is very slow.
7. Avoid Lombardic (short-long dotted) rhythms in your ideally vocal melody.
8. Include at least two different note lengths, for variety.
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 above.
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.
©2011 Ars Nova Software, LLC
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