What does 'xl' mean on a tyre?

[The AJP Griff]

I've been looking into tyre choice for my new wheels.......I know it sounds obvious,'XL' must mean extra large,right?But i just dont get it when it comes to tyre sizes.Surely the size quoted is what it will measure,so what does the 'XL' mean then?Which dimension are we talking about here?
Cheers,Mike

[David Godfrey]

http://en.wikipedia.org/wiki/Tire_code

HTH

[The AJP Griff]

Ah-ha!Thanks David

[jon-]

eXtra Load, normally means re enforced carcass which gives you a more positive feeling when cornering (if not a slightly rougher ride)