What's the difference between a ball and a skein? What makes something center-pull? And what on earth is a hank, and why do some yarns come that way? The form in which a yarn is sold is called its putup. Yarn putups can vary wildly, and many times you use them in different ways. Let me explain the most common forms of yarn put-up. Click below to see the full post!
Ball. A ball is just what it sounds like - a round or round-ish, well, ball of yarn. Usually a ball of yarn is not perfectly round, but more oval. If a ball is hand-wound, it might be more spherical. Store-bought balls of yarn can be center-pull or not, depending on how it was wound.
Mushroom puff. A 'mushroom puff' is many people's least favorite yarn putup. They are often used with luxury yarns with very low yardage. They also have a tendency to tangle as you're knitting them. A 'mushroom puff' is basically wound like a small ring of yarn. You can knit from the edge or from the center, but as you knit from the center the puff will become floppy.
Hank. A hank is the most common putup of hand-dyed and luxury yarns. A hank is basically a long loop of yarn. It can be wound with a tool called a niddy noddy, or with a motorized skein winder (or with a chair back, book, or anything long enough that you can wind around). Usually, a skein is wound up onto itself so it looks like a tight twist of yarn. Before you knit with hanked yarn, you have to untwist the hank and wind it into a ball using a ball winder and a swift (or by hand, but it takes a considerable amount of time and risks tangling).
There are several reasons to store yarn in a hank. Firstly, yarn is usually dyed in the hank, so for indie dyers, it's much easier to sell yarn as a hank than to take the time and effort to wind the balls - which would raise the cost of the yarn for the consumer. Secondly, winding yarn into a ball places tension on the yarn that, over time, could unduly stretch it out. So, if you buy some yarn that you're not going to use for a while, it's better to keep it as a hank than to wind it up right away and let it sit.
Center-pull ball. A center-pull ball can be machine wound or hand wound. Center pull balls have both ends of the yarn on the outside of the ball, so you can either knit from the outside in, or the inside out. Knitting from the inside of the ball is very convenient, because it doesn't roll around. If you're dealing with a large ball of finer yarn, however, such as lace, the yarn has a tendency to tangle as you knit through the ball. This is because the center of the ball is being taken out of it, and it collapses on itself. This can result in 'yarn barf', so use caution!
Cone. Cones are used in weaving and machine-knitting. A cone is an easy way to store yarn that's going to be processed on machines. In the context of knitting, many finer yarns originally meant for weaving or commercial knitting are sold to knitters as coned lace yarns. Yarns can also be sold on cones in bulk (by the kilo or more). You can knit right off the cone, or you can use a mechanical skein winder to take the yarn off the cones, into a skein where it can be washed or dyed, and then wind it into a ball.
Finally, a skein is a general term for one unit of yarn. It may be a hank (most commonly), it may be a ball, it may be 50 or 100 grams or smaller or larger. Just one "thing" of yarn.