Twist stable, but is it balanced?
Many people talk about how they want to make balanced plied yarns. That means that a multiple ply yarn doesn't tend to twist one way or the other because the twist in the singles is counteracted by the opposite twist used for plying. It makes it easier to work with for many projects — the most common reason spinners want balanced yarn is to keep the extra twist from distorting knitted fabric. If you fold a loop of yarn back on itself, or make a skein and let it hang freely, it won't twist. But is it really balanced? How do you tell?
A newly spun single yarn will twist on itself to form a balanced two ply. That's a good way to check your yarn while you are spinning or make a small sample, but as the yarn sits this is less and less accurate. If the yarn is more than a few minutes old, you can't count on it twisting itself to a properly balanced yarn. The fibers start to adjust to their new position and after a while tend to stay that way. If you want to hurry the process along, you can make a skein and wash, steam or merely dampen it and dry under tension. This is called "setting the twist." But it will happen just by letting the yarn sit around, too. And if you don't notice, you might think it has less twist than it really does.
This yarn has been hanging around a few months. Winding it on a bobbin keeps it under tension and it has set in that position. I took two strands and allowed them to hang freely and they hardly twisted together at all.
Twist stable, but is it balanced?
But then I dampened part of it with a spray bottle of water and the fibers in that section started to move: the two singles twisted together more tightly. This is what it looked like when I first spun it and allowed it to naturally twist, but that was a long time ago. The moisture lets the fibers relax and they try to return to their original untwisted form. If I plied a whole skein of yarn so it looked like the relaxed part, it would act like it was overtwisted and try to snarl — the skein would twist and it would just look horrible. But I put it in the soapy water to wash and something magic happens: it untwists into a perfect skein!
If you want a balanced yarn, check a small sample so you know what it really looks like before you start to ply. Even if it acts like the worst overtwisted yarn ever, trust your sample. Then skein and wash (or just dampen) to make the fibers relax.