There are a few ways to think of it:
- Suppose you have two singers. Can you give each of them a unique, interesting line, but the two lines sung together still sound harmonious? This isn’t easy, and it gets exponentially harder with three or four singers!
- Suppose you have two singers, one female, one male. Can you write a unique interesting line for each, have it sound harmonious, then swap their lines and still have it sound harmonious?
- Suppose you have two singers. Can you write an interesting line for the first singer, tell the second singer to sing the same line but delayed by five seconds, and still have it sound harmonious? What if the second singer starts at a higher pitch?
- What if you tell the second singer to sing the first singer’s melody backwards? What if you tell the second singer to take the first singer’s sheet music, turn it upside down and sing that? What if you tell the second singer to sing the same thing as the first singer, but at twice the speed?
https://www.youtube.com/watch?v=8yfHY22Ew1c