(The word ply needs to be explained. If the wool is 2 ply, then it means the yarn consists of 2 plies. A ply pertains to a single strand of yarn. …) Google gets a red X on that one, You have one piece (strand) of yarn, you add too or ply a second piece of yarn by twisting then together. Ply is not another name for yarn, ply = add.
2 ply wool ( cotton, nylon, poly, etc.) is two strands of yarn twisted together. Single ply is not really a term used in textiles, its more or less a term used to say that nothing has been add to the raw yarn.

("My understanding of ply means that each ply a "single" cannot be broken down into more strands, if you try pulling one of the singlse apart, you get fiber. By two ply I was refering to two singles spun together making for a more dense or heavier fabric appearance. " ) I guess the best way to do this, take a stand of string, take two strand of string and spin them together, you end up with a thicker string but if you twist two or more strands and twist them together you get rope or stronger string.
It’s the number per inch of strands of yarn that make the finished material heavier or lighter. You can have a 2 ply woven material with a tight weave or loose weave or like a net made of rope or string.
Does that make sense?

If that doesn’t work for you I’ll try later, I got a flag to paint before my French customer invades the States looking for me.

Happy Trails!
Tom Martin