Could someone clarify what they are and everything? I'e read the article on Wikipedia, and I still don't quite get how you make them. I don't really get the ratios (like 2:3 and 4:5 etc.)

Also, I heard Meshuggah uses polyrhythms, but I listened to Bleed, and it didn't help clarify it to me either.

Do they always need to be in 4/4? Could I say, make one in 5/8 or whatever?

Thanks in advance.
they can be in whatever time signature that works. For example, that 2:3 ratio you saw could be played as a suggesteed bar of 4/4 over a bar of 6/8. For every two quarter notes in the 4/4 bar, 3 eighth notes exist in the 6/8 bar. try playing one of the rhythms with each hand and you'll see what's going on.
Quote by allislost
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Bleed isn't the best example of polyrhythm from Meshuggah - try New Millennium Cyanide Christ; http://www.youtube.com/watch?v=4A_tSyJBsRQ

Listen to how the guitar, bass and drummer's feet are going DUN DUN DURUNDURUNDURN in god only knows what time signature, while the drummers' hands are playing 4/4 (listen to the hi-hats).
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Quote by Freepower
You're getting confused between polyrhythms and polymeters.

http://en.wikipedia.org/wiki/Polyrhythm - this is pretty good.

Do you follow what's written there?

Yeah, that's what I read off of.

I think I get it now, but I'm at school on my laptop... So I can't watch the Youtube video.

I'll try the thing aether suggested maybe during my next class and see how it goes.

If anyone could provide some more information they think would be useful, it'd be appreciated.