[Pc_Support] i586 vs. i686?

[Mike Webb]

Got a simple question, I think. I regularly see the terms "i586" and 
"i686" in my reading, particularly as part of filenames of Linux ISOs. 

I've known that they are generally two branches of the Pentium (and work-
alike) family of processors, but never had enough curiosity (until now) 
to find out what the dividing line was. I don't know if it's the 
absence/presence of the MMX/3DNow! instructions or some later development.

Would someone be so kind as to tell me what the difference is between an 
i586 and an i686? Thanks.
--
My Groklaw nym is Weeble. It came from the "Spiderweeb" and "Weeb" kind of
nicknames a kid named Webb gets. I never even knew of the Hasbro product 
until after I started using "Weeble". Take note, any Hasbro attorneys.