What does it being twoply have to do with how textured the knit is? i get thick and thin, but how does the number of ply change that
very good question! I'll refer you to jillian moreno's book "yarnitechture", because she illustrates it really well.
basically, since a 2 ply yarn is more elliptical than round, the stitches tend to roll away from each other, which creates (as she says elsewhere in the book) "visual motion". it also leads to softer, less defined colorwork. and while she doesn't bring it up in the book, from my own experience, thick and thin 2 ply tends to have the thick parts stand out a little more, and the result is a much more visually and texturally busy piece of stockinette.
conversely, 3 ply is round. when knit into stockinette it creates a flat, smooth, and much more regular piece of fabric. each stitch is more clearly defined and the piece overall tends to look visually cleaner.
which is often a desirable thing, but i kind of prefer the texture.
(if your library has a copy or if you can find one for sale, by the way, i highly recommend getting it. its got a ton of helpful information about spinning and also some really gorgeous photos)
