Disclaimer: One of the most appealing things about knitting your own clothes is the creative control you have over the materials. Yarn selection seems to boil down to a combination of budget, availability, and personal taste. I rarely knit a project from a pattern in the recommended yarn, with mixed results, though I 110% appreciate and respect the fact that when yarn companies work with a publisher or a designer they are doing so in an effort to promote their product. I know that yarn substitution is part of the joy of the process. That said, Its alarming to see how often people end up disappointed with their garments after making unfortunate yarn substitutions. I want to tread very very carefully here so no links, and no references to specific projects…
Every yarn is different. Fiber content is an obvious difference, it effects the way the knitted fabric behaves, the weight and drape of your yarn can completely change a garment. For example, compare Berroco Ultra Alpaca to Cascade 220. Both are worsted weight yarns, nearly the same yards/skein, and often subbed for one another. They are very close to being interchangeable in terms of gauge but the knitted fabric is completely different.
Even when the fiber content is identical, pay attention to the weight of the skeins. If the number of yards per oz/gm is different, the yarns are different, Cascade 22o does not equal Araucania Nature Wool. Nature wool is a finer, you might get gauge but you won’t get the same fabric or fit.
And then what seems obvious, ‘aran’ and ‘worsted’ are not the same thing. And really really obvious, subbing a worsted yarn in a pattern written for a chunky will require math. Sometimes this is as easy as going up or down a size or two but ‘at your own risk’- don’t blame the pattern if you’re not happy with the result.
I’ve been reading project notes on Ravelry lately. Not just notes on projects from my patterns, but from lots of different designers. I began thinking about (pattern) writing style, about garment lengths and widths, and how I often don’t want as much waist shaping as other people do, but got sidetracked when I started reading for information that identified what made projects unsuccessful. It is amazing how often you read that someone is disappointed in the too long/short/skinny/wide garment they just made, and they substituted the yarn.
Please read both the schematics and the swatches carefully. Measure all the time, both the knitting and the body its supposed to go on. Own your knitting, and adjust as necessary.