Plimouth Embroider's Story had an intriguing paragraph today at http://www.plimoth.org/embroidery-blog/2008/05/29/spin-span-spun/

"Ah, another instance of S and Z.
In embroidery, we see the S and Z as the differentiation between the Stem Stitch and the Outline Stitch.
Depending on the direction you make your stitch it creates a twisted border that makes an “S” or a “Z”.
I can never remember which is which, but I believe the “S”tem stitch makes the S and the Outline stitch makes the Z.
In practice, most people interchange them without differentiation- but technically there is a difference. "

I was after something that explained this to me in simpler terms. I talked to Mary Corbet of NeedlenThread and I also went on a Google Hunt. I thought I'd find out more about S and Z twist while I was there. Who knew it mattered in *practical* terms ?!!?

• From the wikipedia article on Yarn http://en.wikipedia.org/wiki/Yarn

• From :

Brazilian Style Embroidery - From Adaption to Obsession
http://www.needlepoint.org/Archives/01-10/brazilian.php

"One fact we should take into consideration is the twist of the fibers we plan to use.

This is important in the success of the wraps required in some of the stitches. Brazilian rayons and some silks have a counter-clockwise or "Z" twist, while most other yarns, threads and flosses have an "S", twist.

It is very important to be able to recognize this twist because if the wrap goes against the built-in twist, the fiber components will separate as it is wrapped.

One way to identify the twist direction, after ascertaining the grain, is to take the fiber in the left hand between the thumb and index finger.

Twist the fiber.

If the twist tightens when rolled to the right, it is an "S"- twist.

If the twist tightens when rolled to the left, it is a "Z"' twist."

(also from this article)........Almost every one has tried a bullion, a very old stitch dating from Biblical times.

Don't forget the twist. A right-handed stitcher, using a "Z" twist will wrap toward the body and away from the body if using an "S" twist. The left-handed stitcher will do the opposite

The article also mentions consideration of the thread twist in doing French Knots (and other Brazilian stitches)

• Bullion Knots http://heritageshoppe.com/heritage/stitches/bullion.html

has a picture to show what goes wrong if a Bullion Knot is twisted against the thread's twist.

• An article that talks about the point Plimouth raised about Stem/Outline stitch is

Stem and Outline Stitches - third paragraph http://www.heritageshoppe.com/heritage/stitches/stemstitch.html

(and I really like the entire article on stem/outline stitch and it's different approaches and use depending on the approach used).

• Another interesting article I found on the way was on the Grain of embroidery thread - http://www.heritageshoppe.com/heritage/essays/threadgrain.html

Note from this :-

"Embroidery floss is normally S-Twist. One exception is rayon threads, which are Z-twist."

I've also read that Brazilian threads are normally Z-twist.

From http://heritageshoppe.com/heritage/stitches/bullion.html

"Sewing thread, some silk threads and rayon threads are all Z twist.

Cotton floss, cordonnet, pearls and other cotton needlework threads are S twist."

• I also have a feeling that, (in the case of S twist, which is all I've ever used) left handed embroiderers have their thread tending to untwist, whilst right handers have their thread twist tighter, as the needle twists slightly each time it goes through the canvas. (And this is why they need to let their needles dangle and spin, to untwist the thread.)
  

Actually, I have both problems, but mostly the thread untwisting from it's plies.

• and lastly, the mystery of why I found most of this information at http://heritageshoppe.com