Here are some helpful places for you to check out on this not so scarey subject once you get the facts. I use Yarndex.com often for the latest info on yarn and yarn availablity. If you have a knitting teacher or experienced knitter nearby, ask them to give you some help, especially if you are new to knitting.
Substituting Yarn from Knit Simple
Yarn Info Finder from Elann
Need to Substitute Yarn from Jimmy Bean's Wool
How to Substitute Yarn in a Knitting Pattern at ehow.com
Always remember to check your gauge, choose wisely and you'll be knitting and crocheting with no fears and no headaches in 2012.
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