To make a plied yarn, spin two or more singles together with a reverse twist. If you spun the singles to the right (Z-twist), then you'll need to spin your plied yarn to the left (S-twist). We use the word "singles" to refer to a yarn with a single twist; a plied yarn is yarn with two directions of twist, and a cabled yarn has three directions of twist (or more). Learning how to ply yarn makes the finished plied yarn stronger, more stable, and more consistent. It averages out the diameters of the singles as they are plied together. The more singles used to make a plied yarn, the more even the resulting yarn will be.
When a plied yarn is made, the twist enters all the singles at once. For instance, when a 4-ply yarn is made, all of the singles are held together as the twist enters the yarn. A yarn made from two 2-plies twisted together is called a 4-strand cable and feels and reacts differently than a simple 4-ply yarn when it is worked into cloth. To ply yarn, start by spinning the amount of singles you want to ply together all in the same direction.
Best Yarn Storage Ideas
Once you have spun enough singles, the bobbins are then placed on a holding device such as a lazy kate for plying yarn. Many spinning wheels have storage racks or rods for bobbins; however, unless the wheel is designed with a vertical plying frame, such as Louet and Schacht Lady Bug wheels, the odd angles make it hard to do a good job. If you don't have a kate, they are easy enough to make by poking holes through both sides of a shoebox with knitting needles and suspending your bobbins on the needles. When you put you bobbins on any type of kate, make sure the singles are pulling off in the same direction.
How to Ply Yarn Step-by-Step
Tie the number of singles you are plying together onto a leader and organize your singles for plying. There is no set way to ply yarns. Here is the way Judith MacKenzie suggests in her article "The Art of Plying" in Summer 2006 issue of Spin-Off, which is also included in the Judith MacKenzie Collection eBook.
Take your drafting hand and place your middle finger (and ring, and pinky, finger for more plies) under the threads pushing them through the "V's" the threads form; the palm of your hand should be facing you. These fingers will make sure the twist enters the threads at the same time. This leaves your thumb and forefinger free to pinch and draft as they normally would when you are spinning short draw. If you need to let go, let with this hand. The other hand is the tensioning hand and is the one that keep everything in order. Keep the tensioning hand in one place while using it to hold the yarns apart as they come of the bobbins. Don't move the tensioning hand back and forth.
Start spinning in the opposite direction to which the singles were spun. Using your thumb and forefinger of your drafting hand, grasp the singles and pull them forward toward the wheel's orifice. When your fingers reach the orifice, loosen your grip draw them smoothly back along the yarn toward the tensioning hand. The twist will follow your fingers back. When you reach your back hand, pinch your thumb and forefinger around the yarn so the twist can't go any farther.
Pull forward, letting the plied yarn feed onto the bobbin. The faster you move your fingers back and forth, the less twist you will have in your yarn, the slower the more twist. Repeat this process until you are done plying all your yarn together. Add new bobbins by tying them on or grafting the ends together.
Best of luck learning how to ply yarn!