If you've ever tried to tune a guitar using harmonics, you find that once you get a couple of strings tuned to each other, to tune the rest, you start having to make compromises, otherwise, you have a guitar that isn't quite in tune. Some chords sound okay, but others, not quite. And the further you get away from the key you tuned it in (say you tuned it in C and now you want to play something in E), the further out of tune it is.

That's what Equal Temperament is all about.

Don Firth