The other day I finished knitting a sock and needed to cut off the yarn. I was a passenger in the car and had forgotten to bring scissors. So I broke it. That is, I tried to break it. With my bare hands.
I've broken knitting yarn before, even yarn with acrylic in it. But this sock yarn would not budge. So I untwisted the yarn enough to get at the 3 single plies. Individually, each of the singles broke with practically a feather touch.
I have always known that singles yarns could be problematic on the warp because they are not as strong as plied yard. And even more problematic would be handspun singles, especially if the handspun is softly spun. I have known for a long time that a two-ply yarn is stronger than a singles. I have known for a long time that a three-ply is stronger yet. I had learned these things when I was weaving with linen and when I started spinning. But I didn't really "know" any of this until I tried to break the sock yarn with my bare hands.
Now I really know in my bones why my plied handspun does not break when I use it as warp.