OK, this is probably a really stupid question, but...how do you know what yarn you can substitute for the yarn called for in a pattern? I'm asking this not for immediate practical application, but just for future reference.
this can help... You can look up the yarn suggested, and then click on parts of the description to get comparable yarns.
For example, the yarn suggested for "Baby's First Tattoo" (sweater in SNB nation) is Rowan 4-ply Soft: http://www.yarndex.com/yarn.cfm?yarn_id=130 I want to make it with a similar gauge yarn, but in something washable and yet lightweight for here in Phoenix. If I click on "gauge" I get this list: http://www.yarndex.com/yarn_by_gauge.cfm?info_id=28 where I find Rowan makes a similar cotton yarn. There is another brand with an acrylic, but it's not available in the colors I need. And so on.
Helpful? You want to substitute a yarn of the same gauge, but also you want something with a comparable feel. I mean I could find WIRE with a similar gauge, but I wouldn't want to knit a baby sweater out of it.
But, there are also things like metallic or ribbon yarns that might not "drape" the same or have other qualities I don't want.
Also, if you have a friendly LYS, they can be a wealth of information. I wanted to make the "Wheelie" from knitty, but they didn't have the yarn the pattern called for. So, I showed them the pattern and they knew a perfect substitution, and how much of it I would need, and were a pleasure to work with. For Arizona mamas - the Fiber Factory in Mesa! I drive nearly 45 minutes to get down there.