During yesterday’s class, there was a question about S-twist and Z-twist yarns and why one-ply yarns often are spun in one direction and when turned into two-ply yarns are spun in the opposite direction. I gave a simple answer of because it prevents unraveling, but here is a better visual explanation why, as explained by the Khipu Database Project.
A certain amount of twist is needed to hold the fibers together. Twist over that amount resides in the thread in the form of energy. A freshly spun singles thread almost always has extra energy, which makes the yarn want to kink back on itself and form snarls. One way to counteract this is to ply two yarns with the same twist together in the direction opposite that in which they were originally spun. That is, two S-yarns would be spun together in the Z direction; this operation is called plying, and each S-yarn is called a ply. The Z-twist of the plying operation holds the two yarns together and balances out the S-twist in the singles so that the finished yarn is stable and has no tendency to kink or snarl.