Plying adds or removes twist from a single as if it were being twisted alone. But since there are two or more strands being twisted together, the friction between fibers holds the yarn together. Even in a perfectly balanced two ply yarn, where the fibers in each single have close to zero net twist, the spiral path of the two yarns around each other holds it together. Plying complex yarns also adds or removes twist from the component elements, which may themselves be plied yarns.
An unbalanced plied yarn does not have the necessary amount of plying twist to exactly counterbalance the singles twist. Unless it is held under tension or treated to set the fiber arrangement, it will naturally twist to compensate, particularly when it is later washed. A balanced yarn does not tend to twist in either direction. It is theoretically possible to calculate the exact plying twist required for any balanced yarn but samples are much more practical, particularly for anything more complex than a simple even two ply. Freshly spun singles, if allowed to twist freely, will create a balanced yarn but most kinds of fibers start to lose this ability within minutes. Instead, wet a sample of the plied yarn to relax the fibers and let it hang freely to determine the balanced ply. If it twists in either direction, the original yarn is not balanced. A skein of unbalanced yarn will also twist. Flax fiber is an exception, the correct twist cannot be determined from wet yarn because the fiber itself tends to twist when wet.
Some fabric structures are more affected by unbalanced yarn than others. If the yarn is relatively free to move around in the finished fabric, an unbalanced yarn may distort the regular structure. This can be a problem, or it can be a design feature.
Woven fabric is generally unaffected, except in some novelty weaves using long floats. Amount of twist or direction of twist in single ply yarns are sometimes used as design elements, but a plied yarn must have a large amount of unbalanced twist to affect fabric structure. Tablet weaving creates unbalanced twist, but the firm warp-faced fabric is not distorted.
Knitting is where most handspinners are concerned about balanced yarns. Knitted fabric will distort if the yarn is too far from balanced. However, some pattern stitches tolerate it better than others and it is occasionally used as a design element. Large netting and hairpin lace also require balanced yarns because they have long floats and large open spaces. Some lace techniques like bobbin lace usually blocked and starched, they can tolerate some excess twist but other openwork techniques are more affected.
For many uses, the concern is more in yarn handling than in how the twist affects the finished fabric. Crochet can tolerate a modest amount of excess twist because the finished fabric is firm, but too much creates a yarn that is difficult to work with. This is why the twist in a single ply yarn must be set by wet blocking for weaving. This is also done for unbalanced plied yarns. Techniques that use strongly unbalanced plied yarns keep the yarn under control with constant tension.
Maximum 36 points
This was scored in three parts, maximum 12 for each item. Examiner 1 gave 10, 10, and 12 and Examiner 2 12 each.
Examiner 1 had a few comments on this. For the first section, that it is not friction that holds the plies together. I think the substance of my description is still valid but I could have explained it better. If the final plied yarn has zero net twist remaining in the singles (because they are twisted around each other) then something has to be holding the individual fibers in place. The twisting of the two elements is somewhat like braiding, there isn't a knot but each twist acts to compress the singles together. This increases the contact between the fibers. There may be a better way of describing this than in terms of friction, but it's been a looooong time since that Engineering Mechanics class. The second comment was on balanced twist for flax, something I mention but don't actually discuss.