I am a bit slow on the uptake but after thinking what Tamar was saying about store bought plied yarns. I realize that when you turn a bit of yarn with a Z twist up side down, it still shows a Z twist. Just to make sure I had that right I looked at it with a magnifying glass. At the same time I realized that you can also look at it and see the diagonal running from the top right to the bottom left....making it a Z twist. The same applies for a S twist where the diagonal runs from the top left to the bottom right. Z stays Z, and S stays S.
have a great day, I am listening to a great Handel program on CBC, and making my Christmas cards
Els