I just sent this to Ben,

Ply: two or more yarns that have been twisted together. The yarn count per inch would make a denser or heaver material but bunting is supposed to be lightweight for flyability, a loose weave. The number of twist adds strength as did mercerizing cotton yarn. A more modern way to explain ply, ply-yarn such as automobile tire yarn, may have 9, 10, 11, etc., ply.

The thickness of yarn depends on the material used to make yarn and how it is spun, loose or tight. Cotton fibers, on the most part are shorter then wool, depending on which breed of sheep it is harvested from.

Happy Trails!
Tom Martin