What is a ply?
If you look into older patterns, you will find that the thickness of the yarn is indicated in plies. In some countries, the ply system is still in use, but it is not customary in the US. The system used in the US classifies yarn in categories such as super fine, fine, light, medium, bulky and super bulky.
The question is, what does ply mean and is it still a viable way to give an estimated gauge or thickness of the yarn?
Plying refers to the process of twisting together two or more strands of spun fiber. Natural fibers (except silk) come in short lengths that are spun into strands; these strands are then twisted or "plied" together to make a stronger, more balanced yarn that is less prone to biasing when knit/crocheted. Therefore, the number of plies refers to the number of strands that are twisted together. Synthetic fibers come in strands too, and the strands are twisted together in the same way as natural fibers.
Years ago, a single strand was a standard thickness, and so the number of plies determined the thickness of the yarn. A 2-ply yarn was a fine yarn, and 3-ply yarn was heavier. Today, strands might be thick or thin; a 2-ply yarn consisting of thick strands might be heavier than a 3-ply yarn consisting of thin strands. When you look at the different types of yarn available today, you will see that the ply system does not work anymore. Using the system offered by CYCA (Craft Yarn Council of America) at http://www.yarnstandards.com/weight.html will give you an estimation regarding the weight of the yarn. However, the best way to figure out if the yarn works for a project is making a gauge swatch. For more information about making a gauge swatch, check NUís knitting or crochet libraries.
Nazanin S. Fard