You've got a beautiful pattern you've been meaning to knit for a long time...but you've put it off so long that the original yarn is now discontinued, and even eBay sellers are out of it!  Or, you just don't like the yarn used in the pattern.  Maybe you want a different color.  Maybe the pattern was written for an acrylic yarn, and like me, you've banned acrylic from your stash.  How can you properly substitute yarn?

• Gauge has to be your first consideration.  You can certainly substitute a yarn of a different gauge, but this means you'll have to do lots of pattern recalculations.  If aran or other stitchwork is involved, you may end up with more headaches than the pattern is worth.  Look for yarns in the same gauge as the original yarn.
• Suitability of the yarn to the type of project is what I always choose for my second criteria.   You wouldn't use cashmere to knit a rug, would you?  Or a rough wool like Kureyon for a baby's layette.  Mohairs and angoras are going to shed, so you may not want them for hats, where the shed fibers may get into your eyes.  If it's a pattern for a wearable item, consider the year-round weather for the recipient.  My sister lives in Dallas, and although I'd love to knit her something with one of our wool yarns, there's never a time of year when she could wear it.  One year I knitted a sweater featured in Knitter's which called for worsted yarn worked on size 13 needles.  The original pattern was in wool, but I chose to knit it in cotton.  Unfortunately the original pattern's finished look depended on the ends of yarn felting to the main yarn a little bit.  Since cotton doesn't felt, my sweater's ends keep popping out of the giant loopy stitches. 
• Yardage!  Usually when you substitute within the same gauge, the yardage will be similar.  However, cotton, for example, is heavier than most wools; you will get fewer yards to the gram.  You may need to buy more skeins of yarn than the pattern used to complete the project.
• Color can be very important.  I like to knit aran sweaters, but I do NOT like to use natural colored yarns!  If you have found the perfect yarn but it doesn't come in the color you want, consider dyeing it.  There are many easy dye methods which can bring every color in the spectrum to your work.
• Price is, of course, often the ultimate deciding factor.  However, with the burgeoning amount of internet discount yarn shops, you can often find good bargains that meet most, or all, of your criteria.
• How hard is it to get the yarn you want?  You've found the perfect yarn - everything on the list checks out...except it's discontinued.  You've still found a few places that sell it.  GET MORE THAN YOU NEED.  I can't recommend this strongly enough!  Buy 3 or 4 more skeins than you think you will need.  If you don't, chances are you will need them for something, and by that time, there may be no more yarn available.  If you have yarn left over, sell it on eBay or use it to make a complementary item such as a hat to go with a sweater.  I also recommend purchasing extra if you are buying from an unusual source, like an international vendor or a small store that produces its own yarns :-)

I hope this guide has helped you with the sometimes-tricky problem of yarn substitution!  Please feel free to contact me if you need more specific help.

Donna