Ply is a big part of yarn. Ply is the number of strands that are twisted together to form a piece of yarn. This number is much different than the yarn’s weight. It is important that you do not get them confused.
A long time ago, what type of yarn you used to knit or crochet was measured by plies. Now it is measured by its weight, which, like was already pointed out, is much different than ply. You have to know the weight of the yarn as well have the right size of needle. The diameter of the plies determines the weight of the yarn. The weight, after all, is the measure of the thickness of the yarn.
The more ply that are in the yarn, the stronger it is going to be. For example, it is easy to break one piece of string that is wrapped around your finger, but if you wrapped it around many times it is almost impossible to break. This is what it is like with the yarn. The more ply strands that are in it, the stronger and more durable the yarn will be.
If you are making something that you want to be very durable and strong, you will probably want to use yarn with a lot of ply because it would take a long time to wear it out. The fewer the ply, the easier it will be for it to wear out quickly or fall apart. It is not likely that it would really fall apart; it would just be much easier for it to wear out.
Some yarns will be two ply or four ply…some will even be eight ply or more. The finer the yarn, the fewer ply that will be in it. Very fine yarn equals very few ply. Very bulky yarn equals very many ply. There are six measure of yarn. There is super fine yarn, fine yarn, light yarn, medium yarn, bulky yarn and super bulky yarn.
If you were to unwind a piece of yarn you could count the amount of plies that were in it. Two ply yarn is going to be much thinner than four ply yarn. Thinner yarns will be better to use for certain things than thicker yarns, just as thicker yarns will be easier to use for certain things than thinner yarns.
You will want to use the thicker yarns with more ply on things such as scarves and blankets. This will allow the blankets to be thicker and cozier rather than way too thin. If you tried to use thinner yarn for something such as a blanket it would probably take you forever to finish it. A thicker weight on bigger things allows you to finish things much quicker.
You would use the thinner weights with fewer ply on smaller, more intricate things such as doilies. Things that have much more detail. The smaller yarn allows for much more detail. Some people will make very intricate things such as bookmarks. These are very pretty. They may not seem very useful, but for some people they are very meaningful.
When you are crocheting or knitting ply is something that you do not necessarily need to pay huge attention to unless you are worried about two-ply or four-ply. Patterns do not worry about ply anymore. They deal only with weight, which includes ply. If you are making something that you want to endure, its endurance has probably already been considered and the people who made the pattern have probably called for the best yarn possible for that item.