2 ply, 3 ply, 4 ply-I am confused!

I have my handy dandy little “Standard Yarn Weight System” from YarnStandards.com but nowhere there am I able to find anything listing what weight 2 ply, 3 ply or 4 ply would be. I keep coming up with patterns for preemie hats listing the yarn suggestion being 2 or 3 ply. Can anyone help a girl out?

Try this page - http://yarnforward.com/tension.html

[COLOR=black][U]The Knitter’s Book of Yarn[/U][/COLOR] by Clara Parkes is an [B]excellent[/B] source for explanations about yarn ply, and what they mean. Not only is it a great source of info, it has some [COLOR=red][B]fantastic [/B][/COLOR][COLOR=black]patterns. I borrowed it from my library, but I’m going to actually buy it, it’s that good.[/COLOR]