Although twist inserted into a single during spinning provides a degree of strength, a strong balance yarn is the result of two or more singles twisted or plied together. The weight of a finished yarn is determined by the thickness of the single and the number of singles twisted together. Generally we spin the single in the S direction and then remove a bit of the twist when we ply in the Z direction.
The whole business of ply can be a bit confusing and contradictory since a 4 ply yarn will not necessarily have 4 singles but relates to the weight of the yarn and can be measured by how many stitches and how many rows it takes to knit a 100mm square on a given knitting needle size. The industry has many ways of expressing the weight of yarn but is seems the most common and standard system in use is known as tex – the number of meters in one kilogram of finished yarn. 10tex is 10 000meters in 1000grams