Can someone explain Crossfire

[shyy]

I know what Crossfire is actually but I am a bit perplexed because I see all kinds of versions and they all are cross fire. But why is there a X1900 XT and a X1900 Crossfire edition. If they all are crossfire why need a crossfire edition, Or would I have to add a Crossfire edition in order to make it crossfire. Ex: X1900XT with a X1900crossfire edition or can it be 2 X1900XT?
Sorry if I didn't make sense.

 

[RichPLS]

One card has to be a Crossfire edition.

 

[shyy]

Thanks!