Pattern writing is a detailed process. It's important that I record everything I'm doing, as perfectly as possible, so that others know just what to do when following the pattern. However, I can't control for everything. Yarn substitution is one of those things that it's hard for me to control for. I can list the weight of the yarn I used, the brand, the color, the gauge with the hook that the pattern calls for... but if a different yarn is used all bets are off. Yarns of the same weight vary enough in thickness and other qualities that two hats made exactly the same way but in different yarns could be very different from one another.
So, what's the solution? I've written about WPI before, and knowing about that helps a lot. However, there is another tool available for this purpose. Have you heard of Yarnsub? It is a really awesome web tool for helping you find yarns that are similar to the yarn called for by the pattern you're following. How does it work? Let me show you!
Let's say my pattern calls for Cascade Yarns Cherub Aran (which I love). What if that yarn isn't readily available in your area and you'd like to start working on your project today? Perhaps you'd like to see if something in your existing stash will work. Well, let's plug it into Yarnsub and find some options.
Yarnsub gives me all of the specs for the yarn in question. How handy! Keep in mind that the gauge listed is for knitting. Not to worry! It also provides suggestions.
Right away you get recommendations for similar yarns on a number of different points, including gauge, fiber content, and even notes about the qualities of the finished project.
So many options! Seriously, I love this tool. It can also help you find less or more expensive yarn options, as well as help you to learn about new yarns and yarn in general. Try it out!