About plies
Knitters rarely use a single ply of yarn. In the textile arts, spinning is done by taking 2 or more strands that each have a twist to them and putting them together. The strands are twisted together in the opposite direction of the originally spun single ply.
In addition, by using multiply plies, manufacturers can create the wonderful yarns that we love.
Multiple colors and multiple fibers can be spun together. various techniques can be applied. An example is boucle yarns where one strand is held loosely and allowed to make loops on the other yarn while plying.
Although the number of plies is interesting, the weight of the singles that make up the yarn is what determines the weight of the yarn.
2/24 = 2 plies of 24 weight singles (spun 24 times)
2/18 = 2 plies of 18 weight singles
4/8 = 4 plies of 8 weight singles