Patterns tend to outlive yarns. Have you even found a pattern you want to knit but can't find the recommended yarn? The yarn may no longer be commercially available, or not available in your country, or your local yarn shop may not sell it, or it's too expensive. Then there's always the possibility that you just don't like it and want to knit with something else.
Substituting yarn can be done with great success, but there's more to it than finding another yarn of the same weight. You need to factor in the fiber, ply, weight, superwash/non-superwash, and more. This is where the experts come in.
Yarnsub is my #1 source for finding alternate yarns. It not only searches their database for similar yarns, but it ranks them from the closest match to the least close match. And ... it tells you why! It looks at ply, gauge, fiber content, weight, yardage per ball, even color selections. This matters because some factors may be more important to you on a project than others.
If you're knitting a summer top that calls for 100% cotton yarn, you may welcome suggestions that offer other warm-weather blends like linen or bamboo. So fiber differences are ok!
If you absolutely want to match the fiber exactly, but the gauge is falling between DK and Sport weight, you may want options for yarns in either weight.
then enter your yarn in the search box.
Here is what we get:
Right at the top, it shows that this yarn has been discontinued, then lists all of the yarn details (which also makes Yarnsub a good place to look up properties of a yarn). Then it lists substitutions:
Since Nettle Grove was such an usual blend, there are no yarns that match the exact fiber. Yarnsub starts with telling you what matches exactly, then lists the differences.
I love the little info tips at the bottom that translate the differences into what it means to me like "this yarn will feel warmer to wear than the original."
The database isn't 100% complete, but with over 12,000 yarns, it's pretty comprehensive. It saves time. It opens up my world to yarns that I may not have known before. And it reminds me to consider all of the factors like ply.