Fiber craft patterns always give a yarn weight. I've seen things like medium and light, but also "worsted yarn" or "DK yarn".
What does this mean?
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Yarn weight is basically the thickness of a given yarn. There are several ways to refer to yarn weight, but the category, the number of plies, the number of wraps per inch and the gauge range (for both knitting and crocheting) are the most commonly used. The weight of your yarn will affect the size of your finished product, and the size of the needles or hook you will have the most ease to work with. You will always it on your skein package.
There are 8 categories of yarn as defined by the Craft Yarn Council: Lace, Super Fine, Fine, Light, Medium, Bulky, Super Bulky and Jumbo. Each category refers to a specific range of the measurements methods. Some categories will sometimes be substituted for a type of yarn, like Worsted for Medium or DK (Double Knitting) for Light.
The number of plies is nowadays probably the least consistent way to talk about yarn weight. Back in the day when knitting was done with actual sheep wool, yarn was measured by the number of single wool yarns in the finished yarn, but with synthetic yarn, it's more difficult to be consistent. For example, a 5 ply yarn in Australia will be called a 6 ply yarn in Germany, but not be standardized in the UK. Yarns below 5 ply are generally pretty consistent though.
Wraps per inch (wpi) is the the number of times you can fit a yarn within an inch. The thicker the yarn is, the lower its wpi will be.
The gauge range is the number of stitches you can make in a 4 inch or 10 centimeter swatch. This measurement is always double, once for knitting and once for crocheting.
Here is a table I made: