The right kind of yarn is essential for any knitting or crocheting project, since the wrong yarn for a particular pattern will usually make it either difficult or impossible to get the correct results. One of the factors to consider when choosing yarn is the ply. Essentially, yarn ply refers to the number of strands that make up the yarn. During the manufacturing process, different strands are twisted together. If a crafter untwists these strands, she can see how many plies make up the thread. For example, two-ply yarn has two separate strands, while and three-ply yarn has three.
A good rule of thumb is that yarn with a higher ply count results in more stitch definition. So, if a knitter wants to see cables and texture in her final project, higher ply yarn is a good choice. People who like a rustic, cozy, or homespun effect may want to choose yarn with a lower ply count. In addition, a lower ply count often blends different colors together, resulting in a very pretty watercolor effect.
One advantage of working with a higher ply yarn is that the act of twisting strands together can help to correct the tendency of the yarn to slant as it is being worked with. When knitting with a lower ply, a crafter might notice that her stitches look more like a lopsided check mark than the neat "V" shape she expected. If the yarn is extremely unbalanced, the entire swatch may even seem to tilt to one direction.
Another factor to consider when choosing the yarn ply for a project is durability. Generally, yarn with a higher ply count is more durable. The process used to create the plies keeps the fibers tightly wound together. As a result, projects are less likely to pill, shed, or wear thin when washed.
It might seem like yarn ply would be directly related to yarn weight, but a yarn with four plies is not necessarily thicker than one with only two. If the individual strands are very thin, even having several strands twisted together might not result in a very thick yarn. One simple way to measure the weight of yarn is to wrap it around a ruler, then count the number of wraps per inch (2.54 cm). Yarn weight is also listed on the label, ranging from 1 (super fine) to 6 (super bulky).