Yarn Twists
Most yarn is composed of combined fiber strands called "plys".
To make a cohesive piece of yarn these plys are twisted together. However there are two directions which they can be twisted, and these are called S-Twist and Z-Twist.
Here is a diagram showing the visible differences.
As you can see S-Twist goes upward to the left and Z-Twist goes upward to the right. You can often be able to tell by just looking at the yarn and direction of the plys.
Another way of telling is by twisting the yarn clockwise (to the right). S-Twist yarn will get further bunched and a tighter twist, while Z-Twist yarn will get looser and the plys will split. The above diagram shows how it will look.
Most yarn is S-Twist yarn. This twist of yarn is ideal for knitting. While knitting the twist of the stitches reinforce the twist of the yarn and improves stitch definition.
Unfortunately, the same can't be said for crochet. While doing a "yarn over" (used in most stitches), it twists the yarn in the opposite direction and makes it prone to splitting.
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