Every time you spin a yarn, you are adding twist into a series of parallel fibers.
The direction that twist is applied changes the angle of those fibers. Your yarn will either twist to the left or to the right- either a s or z twist. How compactly those twists are stacked on top of each other (the amount of twists per inch) indicates twist angle, and how much twist has entered the yarn. Measuring the twist angle throughout the spinning or plying process can help ensure a more consistent yarn (that being said, I don’t measure twist angle as a general rule since I like my yarns to be more ‘organic’ than machine or even wheel spun yarns). Basically, the more twist that is in the yarn, the tighter the angle between the twists (5 degrees versus 10 degrees, and so on).
How to tell direction
The easiest way to tell the direction of twist is to compare which way the twists are leaning.
If they are leaning to the left then they are a S twist.
If they are leaning to the right then they are a Z twist.
Why is direction important?
Theoretically speaking, the direction of twist can make a difference for spinning technique (worsted v. woolen) and the intended usage of the yarn (crocheting v. knitting v. weaving). PERSONALLY i haven’t found this to be the case, with the exception of worsted versus woolen (but more on that later, and that has more to do with my tendency to smoosh air out of the fiber when i highly twist a single).
However, where direction is extremely important is in plying. When plying you’re trying to balance out a yarn so it hangs straight. One of the easiest ways of checking energy in a skein is to see if it twists on itself. If it doesn’t, it’s well balanced. When plying you want to spin in the opposite direction from the initial spinning- so in other words, S singles are spun into a Z yarn.
How to tell twist direction
The easiest way to tell twist direction other than examining a single is to keep in mind which direction you normally spin.
I spin counterclockwise as a general rule- I’m self-taught and it’s what felt comfortable at the time, and I haven’t had any reason to change it. So I initially spin S and then ply Z.
Yarn spun counterclockwise will produce a S ply.
Yarn spun clockwise will produce a Z ply.