To clarify the “two strands, S spun, and double-plied Z” directions — wool that is S spun was spun on a wheel moving in the clockwise direction (clockwise from where the spinner sits). Wool that is Z spun is spun in a counterclockwise direction. To ply, you want to go in the opposite direction from how the strands were spun. If you ply in the same direction as the spin, you will get a really hard yarn!

The direction also has to do (historically, anyway) with the type of yarn being made. S spun for woolens; Z spun for worsteds. I always think of it in terms of: Woolen — carded — S spun/Worsted — combed — Z spun. Distinguishing between carding and combing is also a tip as to the breeds of wool being spun.

It would be interesting to see if silk responds differently to S or Z spin. An archaeologist friend sent me an article where a colleague of his proved that flax naturally spins in one direction, and hemp in the other. She was able to use the cordage impressions in pottery shards to determine what the clay had been wrapped with, which absolutely blows me away!

Is this just another way of saying 2-ply or is this a different technique?

I really appretiate all the information you are sharing with us. Yes, I already know a few of the things you write about but I am learning more all the time, and I thank you for it.

This has been an incredible journey, even for those like me who can only watch from the side lines.
CA