Mushroom Spinners Control Tool , Wpi (Wraps Per Inch) Ruler Made From Walnut Or Maple Wood For Yarn Gauge, Shroom Moon And Stars Mushrooms with crescent moon and stars Spinners Control Tool. This WPI (Wraps per Inch) Ruler is made from walnut (dark) or maple (light) wood and is used to identify the gauge of a yarn. Wraps per inch (WPI) is a way to measure the thickness of a yarn. This tool is used by spinners, but is also helpful for knitters and crocheters to determine the weight of the yarn and what the approximate gauge will be. This guide also has lines on the back to measure your yarn against, including lace, fingering, sport, DK, worsted, bulky and super bulky. This yarn guide measures about 3 inches tall by 1.5 inches wide and comes with a brass clasp to hang on your knitting bag.