Layering melodies.

This is a pretty basic question, but I'm self-taught and never really asked before.

In composition, when multiple instruments are playing different melodies at the same time, how can you ensure that they all sound good together?

Do they just have to all be playing in the same key, or is there more to it?

Hmm, tricky question.

Well they don't have to play in the same key to sound good, but they would have to be playing at the same volume so that each part can be heard. Or just a slight difference, depending on how important the part is. So the key isn't the factor here, but volume is. Someone else might give you a more detailed description ;)

The problem is that we don't mean what you mean by "sound good together". You could put almost anything on top of each other and some people might find it "sounds good", whereas others won't.

I'm just going to make the basic assumption now that with "sound good together" you mean "do not conflict" according to common practice harmony/voice leading.

In such traditional music, sure, the first thing that almost always applies is that all instruments are playing in the same key. Furthermore, all melodies follow the same basic harmonic succession. In other words: You might consider what the harmonies are that would fit under your first melody, then think of tones for the second melody that fit into the same harmonic succession. This doesn't always have to be straight notes from the basic triads; It's perfectly common to use dissonant chords inbetween as suspensions, passing tones, etc., but the general "chord progression" stays the same. When both melodies fit to a common harmony you may already have reached your goal. But according to common practice tonality you then also need to consider voice-leading: This basically means the directions the voices move in relation to each other. Certain things are "forbidden" here by traditional rules, such as having both voices move in parallel fifths or octaves. Generally, to give the impression of two independant melodies, you may want to give them differing contours. That means that when one voice is going up the other voice isn't always going up, but sometimes going down, sometimes staying on the same tone, and only sometimes going up as well. Differing rhythms can help a lot too in this respect.

It's impossible to give all the details about how voices/harmonies are set together in common practice tonality however, since there is both a vast amount of theory there, but also great differences between different styles.

And as I said, this only applies if you care about common practice tonality in the first place. You are perfectly free to put any melody on top of any other melody - if it sounds good to you. So really, my advise would just be: Experiment. Try putting notes above other notes and see how it sounds. Then make your conclusions.

Gardner did an excellent job at explaining basic counterpoint (secondary melodies). He is also very correct on the point that contrapuntal (an adjective form of "counterpoint") music can be very advanced and takes a LOT of music theory study).

Put simply, counterpoint is written just as harmony is (which, like gardener said, may be consanant or dissonent). The only differance is that a counterpoint is made to be more proment in comparison to the melody. Its a second melody which is equal to the first whereas a harmony is subserviant to the single melody.

Often times contrapuntal music is also polyrhythmic, (multiple harmonic rhythms as opposed to homophonic, which has a single rhythm that is the same throughout all harmonic parts). This is because a counter rhythm makes the secondary harmonic melody more prominent and therefore more contrapuntal.

The short answer is: counterpoint. But whether it sounds good, is something subjective and it depends a lot on the listener's perception and taste. There are no strict counterpoint "rules" - it all depends on the style, period and composer, so I'd prefer the word guidelines. And I'd say music is contrapuntal to different degrees, not just in a black and white sense.