Worsted weight yarn -- also known as aran, afghan or medium yarn -- may be the most popular yarn for knitting and crocheting. Nice and thick without being too bulky, it's available in numerous fibers, colors and types. Worsted yarn knits up quickly and is suitable for most of your knitting and crocheting patterns.
Worsted yarn is generally any yarn that knits up at a gauge of 16 to 20 stitches over 4 inches of stockinette stitch, or 11 to 14 single crochet stitches over 4 inches. The recommended knitting needle sizes are between U.S. 7 and U.S. 9; recommended hook sizes are between I and K. Use a smaller needle or hook to produce tighter gauge, for a thicker, stronger fabric. For a fabric with more drape, choose a larger needle or hook size.
Worsted weight yarns can be single-ply, or two-, three- or four-ply, with several strands twisted together. Spinners determine the weight and gauge not by the number of stitches used to knit or crochet 4 inches, but by wraps per inch, or WPI. By wrapping a strand of yarn around a special WPI tool, a ruler or even a pencil so each strand is next to the previous, a spinner can figure out what gauge the yarn will be. A worsted yarn has 11 to 13 wraps per inch. Spinners are also concerned about the fiber being used to create the yarn. Worsted yarn is traditionally made with straight, parallel fibers -- specifically wool, though any long fiber can be used now. The fibers are combed so they run in the same direction. These fibers are gathered together and twisted to achieve the appropriate gauge.