When we classify yarn by weight, it has little to do with how heavy a yarn is, but rather how thick is the strand of yarn. The Craft Yarn Council, along with publishers, fiber and needle and hook manufacturers, have created a series of guidelines and symbols to bring uniformity to yarn, needle, and hook labeling to make it easier to select the right materials for a project and complete it successfully.

If your pattern of choice calls for a specific yarn you can use a different yarn, by referring to the yarn weight as a basis for substitution. While most yarns within a yarn weight category are interchangeable, not all are identical. It is imperative to do a gauge swatch with the yarn that you intend to use in a project (especially if its a fitted garment) to be sure that you are matching the gauge of your project pattern.