I take a lot of care in selecting the yarns I use in my designs–in fact, often I start with the yarn and see what it wants to be knitted as. Whenever possible, I strongly recommend using the same yarn for your project. However, I know that’s often unrealistic. You may need or want to knit from stash, or be limited by price or local availability. Whatever your reason, I would like to help you make a good substitution.
Some factors to consider:
Yarn weight. Did I use DK? Worsted? Fingering?
Gauge. I don’t always knit at the recommended gauge for the yarn. So the pattern gauge might suggest a typical DK gauge, but in fact call for a fingering weight yarn knitted loosely (for example).
Fiber composition. Different fibers behave in different ways. Cotton will almost always act like cotton, wool will almost always act like wool, and so on. Try to choose yarn with the same fiber make-up. If you are substituting because of a fiber sensitivity or to make a garment more appropriate for your climate, try to choose a fiber that will behave similarly.
Doubling a thinner yarn to get gauge:
A common misconception among knitters that holding any yarn doubled will get you the approximate gauge of the next size yarn up. This is not accurate! To determine appropriate substitutions with doubled yarn, look at the suggested stitches per inch. If you are doubling your yarn, you need to use one that, used single, gets twice as many stitches per inch as the called-for yarn. For example, sock/fingering weight (8-9 stitches per inch) held double = worsted weight (4-5 stitches per inch).
I know this can all get confusing. If you are uncertain about a yarn substitute for one of my patterns, please email me! I’d love to help you choose. I also have a Ravelry group, Noir Knitters, where you can talk to other knitters and look for advice.