In spinning, there are two types of twist:

1. Z twist, which is clockwise twist. This is twist that is inserted into the fibers when the drive wheel is spinning to the right or clockwise.

2. S twist, which is counterclockwise twist. This twist is inserted when the drive
wheel rotates in a counterclockwise direction or towards the left.

Note: The direction of rotation is from the vantage point of the spinner when seated at the wheel and she or he is ready to start spinning.

Normally, Z twist is used to spin singles and S twist is used for plying. There are exceptions, of course, but for our discussion here we will assume singles will be spun clockwise or Z and plying will be done counterclockwise or S.

Z S

Look at the two letters above and notice that the vertical stroke in each slants in a different direction. These two letters show exactly how the twist in S and Z spun yarns differs. Z twist is called that because angle of the twist matches the vertical stroke in the Z. The same is true for S twist, the twist angle matches up with the vertical stroke of the S.

Why is this important to know? Let's say that you have a skein of plied yarn and it is all kinky and unbalanced. To fix the problem you have to know exactly why it is unbalanced and here is where the S's and the Z's come in. There are only so many ways that a skein can be unbalanced. There is too much or too little twist in the singles or there is too much or too little twist in the plying.

Spin a small amount of fibers Z twist. Make a small skein with this yarn. Now, hold the skein open at both the top and the bottom. Watch the top of the skein closely to see what will happen and let go of the bottom. What happened at the top of the skein is that the left side crossed over the right side. That is a clockwise motion and is active Z twist. Whenever this happens with a skein of yarn, it shows that there is too much active Z twist in the yarn for the skein to be balanced.

Now, spin another little skein but this time spin it counterclockwise or S twist. Hold up the skein as before, watch the top, and let go of the bottom. This time the first movement at the top will be from the right to the left. This is active S twist. Whenever this motion occurs, you know you have too much S twist in your yarn.

Label your little skeins with the twist direction and keep them for reference samples.

It does not matter if the yarn is plied or not. Just hold up the skein and watch for that first direction of motion at the top. If it moves towards the right, there is active Z twist; if it moves to the left then there is active S twist.

Here's a simple and fast way to check for S or Z twist. Take a piece of yarn, 10" to 12" long and tie knots in the ends so the twist won’t run out. Place the strand between your index and middle fingers so it hangs down over your palm. If the twist in the yarn basically matches up to the slant in your lifeline, then its S twist if the strand is in your left hand and Z twist if the strand is
in your right hand.