Sock yarn is often a blend of fibers that make the finished fabric more durable. While regular wool yarn can be used for a sock, many people prefer superwash wool for socks because it can be washed in a washing machine.
Different kinds of socks, for different purposes, can be made with different kinds of yarn. Most sock yarn, however, is yarn developed specifically for that purpose. It is usually lightweight (though it can be heavier) and knit on small needles.
Other good choices for socks include wool blends and just about any kind of yarn with a bit of nylon added for durability.
The best yarns for socks are elastic, because they need to stretch around a foot but then cling to the foot while wearing; resistant to abrasion, since socks rub on the foot, the shoe or the ground as they are worn, and that puts a lot of pressure on the fiber; and moisture management, because feet sweat and socks need to either absorb or wick away that moisture in order to feel (and smell) good.
For example silk is too inelastic to serve as a sock yarn on its own, adding a little silk to other fibers gives a nice shine that makes a pattern a little more feminine. While cotton on its own would easily stretch out of shape, a cotton blend can be a great choice.
Cotton, for instance, is great at wicking away moisture and actually gets stronger when wet, while adding wool brings softness, warmth and better ability to recover from stretching than you'd get with cotton alone.
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