I have heard (read) people mention "tpi".. now I assume that tpi stands for "twists per inch". If that is the case, *how* do you work out how many twists there are per inch of yarn and how do you maintain it?
A: Yes tpi stands for Twists Per Inch. First as a COE recipient and a spinning teacher, I want to clearly say, many excellent spinners do not worry or struggle to understand tpi. They go by visual recognition of what good yarn looks like. And they also make a conscious decision ahead of time whether a certain yarn will be "hard" spun or "soft" spun. These terms refer to the amount of twist that holds the yarn together, whether it is a great amount making a tight, hard yarn or a small amount making a loose, soft yarn.
Consistency in drafting the yarn size and treadling "maintains" the amount of twist.
If you must know the exact amount of tpi, then you must unspin the yarn in question. Fiber choice, fiber diameter, crimp, body, yarn diameter, etc. all effect how the twist looks and feels.