Yarn Weight - 4- Medium | Worsted Weight Yarn. When we talk about yarn weight, what we mean is the thickness of the yarn. The Craft Yarn Council is a representative of yarn companies and they came up with a way to standardize yarn by thickness and to standardize knitting needles and crochet hooks. Before size 4 Medium Yarn was standardized, it used to be called Worsted Weight Yarn. It is also known as Afghan Yarn and Aran Yarn. Medium weight, worsted weight yarn, is the most common thickness in knitting and crochet. Yarns of this weight will print the yarn label with the #4 weight symbol and will say "medium". Worsted weight yarn is ideal for all kinds of knit and crochet apparel, accessories, blankets and other home decor items. If you see a pattern that tells you to use a worsted weight yarn, find a size 4 Medium yarn for your project. You can find all the medium weight yarn we carry below.