Unbalanced yarns are often yarns that are spun with extra twist with the idea of plying them together with other overtwisted yarns. When they are plied together in the opposite direction, this extra twist is removed making the new, plied yarn a balanced yarn. Some yarns are never meant to be balanced whether or not they are plied and some look balanced on the cone but aren’t truly balanced.
Identifying unbalanced yarns
To check out a yarn, tale a piece of it about a yard long. Take your sample about a yard or so sway from the cut end. Hold one end of your test sample in one hand and the other end in the other hand. Bring your hands closer to one another so the yarn drapes into a U shape. When you bring your hands very close to one another a balanced yarn retains the U-shape but an unbalanced one will twist on itself forming one strand. This is called the “snarl test.” See Figures.
Some yarns pass this test because they are set in the manufacturing process. To be absolutely sure, put another test piece in very hot water. Unbalanced yarns twist or kink up on themselves.
Another way to look for unbalanced yarns is to see how they act as fringe. Unbalanced yarns have a tendency to roll around themselves. In other words, when they are in a fringe or left to themselves not under tension they tend to automatically forming bunches of yarns twisted around each other looking like ropes.
Plied and single yarns
The strand made up of both halves of the U-shaped yarn is a plied yarn because there are smaller individual yarns twisted together making up one larger one. A plied yarn can have two or more “plies.” (A 4-ply yarn is composed of 4 plies.)
A yarn made of just one ply is called a “singles” yarn. If you untwisted it you would just see fibers, no individual plies.Singles yarns are very often unbalanced.
Taken from Books 2 and 3 and Weaving for Beginners