I was explaining to some friends about why I spin yarn, and the topic of S versus Z and the effects of those spinning techniques on the finished project came up. This was responded to with a number of blank stares.
To put it simply, when the flyer twists the yarn in a clockwise fashion, you're spinning (or plying) Z, and when the flyer twists the yarn in the counter-clockwise fashion, you're spinning (or plying) S.
Here's a picture to help:
Whether this is two plies of yarn we're looking at, or the direction of twist of the fibers in a single, picture yourself holding the near end of those fibers, and something twisting them clockwise or counter-clockwise at the other end. For the purposes of this tutorial, it doesn't matter whether the finished yarn is a single or plied. All that really matters is the last twist put into it. The only difference is with plied yarn it may come unplied a bit, and for a single yarn it may come untwisted (and potentially hinder the integrity of the yarn, but I'll cover how to avoid that).
So how can twist work for or against you while knitting/crocheting/weaving your project?
I'll start with knitting. The yarn goes around the needle in the clockwise direction (generally. Although there are stitches out there that wrap the opposite direction, but let's keep it simple). This will add twist to Z twisted yarn and remove twist from S twisted yarn. Makes sense, right? We're moving the yarn in the opposite direction to which it was last spun, thus taking just a little bit of at last spin out.
On to Crocheting. Again we have the issue of which way the yarn goes around the hook before being pulled through to make a loop. My friend Kim wraps the yarn around from bottom to top (counterclockwise) to pull it through, and I wrap the yarn from top to bottom (clockwise) so our crocheting looks very different. The same principles apply to crocheting as they do to knitting, counterclockwise will remove twist from Z and add twist to S, clockwise will add to Z and remove from S.
So what about weaving? Let's take another look at the picture above. See how the fibers or plies slant to the right or the left? Let's stack them, and make those letters a little smaller:
See how when you stack two sets with the same twist direction (Z-Z here, but the same would apply to S-S), the plies or fibers crisscross each other? Here the texture of the weaving would be distinct. Then when you stack yarns with twist going in the opposite direction, they almost nest together. This fabric will be flatter, less textured.
So what are the ultimate ramifications of this? Two-ply yarn illustrates this the best. Sometimes, when knitting or crocheting with yarn that's 2-ply, it almost ends up seeming like you're just working with two strands held together, because some of the plying twist has come out. Other times, yarn that feels soft and drapey in a skein (its relaxed state) will seem rougher once knitted or crocheted. It's all about the twist. While the small amount of twist added in the act of knitting or crocheting isn't much, it can still have an effect on your final project.
So what to do?
I like to add more twist than I think is necessary to my plied yarns. This is because I knit continental (picking) and my favorite way to spin and ply is to spin Z and ply S. I know that my knitting will take a little bit of the spin out of my plying efforts, so I plan ahead.
When I was spinning yarn for my friend to crochet into a hat, I studies how he crochets. He moves the yarn clockwise, so I spun the singles S and very loosely. I know that the structure of his crochet stitch will add a little twist to the singles, making them stronger, but it should keep enough air in it still to make the hat super warm and soft. If I were spinning yarn for my friend Kim, I would spin singles (or ply) Z, because she moves her yarn counterclockwise.
With handweaving, if you're working with commercial yarns, you're stuck with however they spun the yarn. This is where the art really comes in. You can spin singles that go in the Z direction for warp, and either singles spun in the S direcion for weft, or 2-ply yarn that's spun Z and plied S for weft. Your weaving will lock together like cobblestones. Or, if you want a more textured handwoven piece at the end, make sure all yarn is spun in the same direction so that those plies intentionally do not nest.
I hope this tutorial has been more helpful than confusing, I know I've had fun writing it!
(although it may need more pictures!)