Knitting yarn is composed of twisted strands of fiber, which are known as plies when grouped together. These strands of yarn are twisted together (plied) in the opposite direction to make a thicker yarn. Depending on the direction of this final twist, the yarn will have either s-twist (the threads appear to go "up" to the left) or z-twist (to the right). For a single ply yarn, the direction of the final twist is the same as its original twist.
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