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Counterpoint is not difficult to learn, but you can write terrible music that follows all the rules, just as terrible words can be stuck into sonnet form. A rules system may help, if anything to help imitate an existing style, or to provide guidance for the noob. Some of the rules are pretty academic, especially if there is an exam to pass. The following is a small introduction to counterpoint, and assumes you know enough about music theory (and lilypond notation) for this to make sense.
Given an arbitrary chord, say <d f a d> and an arbitrary next note of c in the soprano voice,
one may want some other notes to go along with that C. Choices here might be limited by a rules system to <c e g>, <a c e>, or <f a c>, possibly in first (but not second!) inversion, so also <e g c>, <c e a>, <a c f>. Various arrangements of the upper notes are possible, though here the bass note would need to be one of c, a, f, e. The bass and soprano are the most important; the middle voices tend to be lost in the mix. A bad choice for counterpoint would be <c e g c> as the voices are not moving independently enough. In particular there is parallel motion at the octave and the fifth, which are generally forbidden.
This isn't to say that <d a'> to <c g'> is illegal; Bach uses parallel fifths in some of his chorales, though only where no better option is available, and the usage is rare.
It is better to move by contrary motion, so given a soprano descent from D to C, one would expect the other voices to rise or hold steady. Motion by leaps is generally frowned on. Voice spacing will be an issue if the voices are too close, or certain moves may simply not be possible and one will have to backtrack some amount and try something else. Voices are generally not allowed to cross. These rules originate with the choir where the soprano, alto, tenor, and bass generally stick to their own registers, though there is some overlap. An electronic device need not stick to choir voicing rules. Anyways, a better progression might look like the following.
A wider voice spacing would really help here, or maybe the soprano should go up so the other voices can go down. Also it may be useful to put each voice on a distinct staff so that it is easier to tell exactly how a voice moves.
The parallel motion <d e> to <f g> is permitted; thirds or sixths are generally okay to run in parallel—but not for too long, and there can be various exceptions as to what is allowed. Another test is to listen to the result. Also note how the bass voice has been moved an octave down. A wider spacing for the lower voices is generally acceptable; close voiced bass notes tend to sound muddy. Moving the voices too far apart however may create too thin a texture.
One way to practice counterpoint is to start with some melodic or harmonic line, and then build counterpoint in other voices. Two, three, or four or more voices may be used, though as the number of voices increase the number of rules that need to be broken will go up. There are only so many ways voices can move, so with many voices either voices will need to go silent or move in with an otherwise illegal parallel with another voice.
Another way to practice counterpoint is to study existing examples. Bach is typical here, e.g. his Chorales or Inventions. Yet another way would be to find a teacher who actually knows this stuff.
Smaller notes can be used to avoid plodding along by quarter or half notes the entire time, though where the counterpoint stops and ornamentation or texture begins may be difficult to say. A notable form here is the suspension, where a note from a previous chord is held over and then resolved in the next note.
Note how the G is held and resolves to F in the alto line, and C to B in the soprano line; these are suspensions. Notes have also been doubled to change the texture, and some voices use passing dissonances or leap to another note of the chord. More information on suspensions and dissonances can be found in the following document.
The elaboration does have a harmonic plan (descent line in bass, somewhat hidden through first inversion chords), though with counterpoint one is supposed to be finding good horizontal (melodic) lines. It may help to listen to (or sing) the lines in isolation to see where they need work; this is another reason why the voices should be kept distinct.
The lilypond files and a build script for the above images can be found in the following tarball.
tags #counterpoint
