Yarn purchased from yarn stores or from individual yarn makers comes in a lot of different styles and configurations, but there are some basics that usually hold true, including the fact that yarn is typically made up of plies, or should I say at least one "ply."
Plies are the individual strands of yarn that are worked together to form a plied yarn. A single strand being sold or worked with on it's own isn't usually called a ply, though, it's known as a singles, because calling it a one-ply yarn doesn't make much sense.
Multiple plies can be used to create a knitting yarn, and while some countries use the number of plies as a shorthand for the thickness of the yarn (a two-ply would be thinner than an eight-ply, for instance), in most cases the number of plies has nothing to do with the thickness of the finished yarn. You can have a very bulky two-ply yarn or an extremely thin four-ply yarn depending on how the individual singles were produced.
To ply yarn, individual singles are spun together with the twist worked in the opposite direction from how the singles were spun. This gives the yarn much more strength, durability and consistency than is seen in a singles yarn.
All About Singles
Singles have much more capacity for weakness -- sometimes tugging to hard can pull the fiber apart from itself -- and visible signs of wear than plied yarns. They're more susceptible to pilling, which is when little bits of fiber sneak out of the knit fabric and make little wads (known as pills) of fiber on the knit surface.
Singles yarns can also bias when you knit them. The plying together of two threads spun in different directions gives balance to the whole. With just one direction of spin in your singles, your stitches may lean as you knit.
Clara Parkes, in her , says smooth, tightly spun yarns (such as silk and cotton) are more likely to bias than fuzzier yarns that aren't spun as tightly.
One way to combat bias, she says, is to make sure you use as stitch pattern that incorporates lots of knits and purls.
Piling on the Plies
If two singles are spun together you get a two-ply yarn, which is stronger, springier and more textured than a singles.
Add another ply and you have a three-ply yarn, and so on, with each new ply adding both strength and density. Parkes says three-ply yarns are perfect for projects that get a lot of wear and abrasion, like sock.
Plied yarns themselves can also be plied together, producing what is known as a cable yarn. To make a cabled yarn, singles are spun with the twist in one direction, they are plied in the opposite direction and the plies are plied together in the same direction that the singles were spun, adding even more strength to the final yarn. This makes a very stable yarn with great stitch definition (it's perfect for knitting cables, for instance).
Cabled yarns can also be made with plies that aren't all twisted the same way for a different effect known as boucle.
Understanding how a yarn is made and how many plies it has is important even if you never spin your own yarn, because different numbers of plies -- and plying methods -- can make a big difference in how the finished yarn turns out and how your project looks, feels and wears.