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Craft yarns are usually measured by weight  and this is done at a standard temperature and humidity, so the length of yarn on a ball varies because of the thickness of a strand and the different fiber types. Hand craft yarn has labels on them which include information on the gauge of the yarn which tells you how many stitches and rows are produced per inch or centimeter depending on your knitting needle or crochet hook.

Wools quality is determined by it diameter, crimp, yield, color and strength. Its diameter being the most important of these because this will determine its quality and price. The finer the wool, the softer it will be, so this would usually be used for garments and the coarser wools would be used for outerwear and rugs.