Often I am asked what the various yarn notations mean. It’s a common question for students as they learn to weave, and want to buy yarn. Unfortunately, knowing the yarn notation doesn’t automatically mean you know what you want, but it helps understand the basis of the system, and that the numbers are relative to the size (diameter) of the yarn…..sort of.
Here is a good explanation of yarn count, also known as the yarn notation system. I have not checked to see if it’s accurate, but the information is not readily available, so maybe whoever sets the rules, gets to make them.