Substitution-Doubling your yarn
Doubled Fingering Below, Sport Above
Rare is the knitter who chooses the exact yarn as recommend by a pattern. Substituting a yarn can happen for many reasons: 1) you have a different yarn in your stash 2) you don't like the colors in the recommended yarn 3) you are allergic to the recommended yarn 4) you have another yarn in your cart needing an excuse to purchase, etc. The reasons for substitution are as varied as the day is long. And substitution is generally a pretty easy business, unless the yarn you want to use is a different gauge, and then it gets tricky. Sure subbing Aran or Worsted is no cause for puzzlement but say you want to trade Fingering for Sport or DK for Worsted or, even crazier, fingering for DK. Don't get your underwear in a twist-it's easy if you know how.
Doubled Sport below, DK Above
Subbing yarn is all in the rule of doubling and as long as you know the hierarchy of yarn gauges-Lace, Fingering, Sport, DK, Worsted, Bulky & Chunky- you'll be fine. Each hierarchy level can be reached by doubling the size below. For example, to trade a Fingering weight yarn for a Sport weight yarn you will need to double the fingering yarn, so you will generally need double the yardage. To trade Sport for DK, double the Sport weight. For DK to Worsted, double the DK, and so on up the hierarchy. This also makes it easier to make great leaps in the hierarchy: to sub Fingering for DK, you will need 4 strands of fingering. This is because you need Sport weight doubled for DK and Fingering doubled for Sport so that makes 4 strands of Fingering weight. It can get to be a handful if you trade Fingering for Chunky so I would not recommend jumping up the hierarchy more than 2 steps.
To knit with more than one strand is just as easy as one strand; just hold all your strands as you would one (just check to make sure you loop them all in each stitch until you gain confidence. Doubling is also a great way to create your own variegated colors (by holding different colors) or add tone-on-tone variations or add a punch of color or texture (by holding 2 strands of the same size by different types of yarn. i.e. 2 sport yarns, 1 wool and 1 silk) to a simple pattern. Using this technique can also flesh out your stash, give you new ideas and encourage you to purchase that special yarn that you would otherwise have no idea how to knit up. I love using this with Mohair, since I am not a big fan of mohair given its lightness and fuzziness that loves to disobey my every wish; I combine it with wool to add softness and ease of knitting. This it does something special to both the mohair and the wool. This is one of my favorite tricks for feisty yarns!