It is a pretty common mistake for new knitters when it comes the issue of yarn ply. For example, NM1/26 refers NM26 in one ply and NM2/26 means 2 ply yarn which is equal to 2*NM26. Yarn count expresses the thickness of the yarn and when choosing the yarn ply for a project, 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 is true that you cant necessarily tell what the weight of a yarn is from how many plies it has. You could have a very thin yarn made from six very, very thin plies of yarn. Or, you could have a thick yarn that is made from a single ply! It's definitely not a precise way to tell what weight a yarn is.
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 manucturing 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.
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.