WPI = wraps per inch

Dear users and authors,
not everyone is familiar with WPI (wraps per inch) yet.
Would you be so kind as to share all the information and insights you have on this topic here in this thread? Thank you. 

I think this is a very “useful” way to describe yarn thickness. After all, different fiber blends can have the same yardage but still vary in thickness.
In one of the swap packages, I adopted a yarn that had a very high yardage per 50 g but was still very thick, so to find a suitable pattern, I then measured the WPI.
  Take a pencil or something similar (the thickness doesn’t matter) and mark off a section of one inch—that is, 2.5 cm—with tape. Then wrap the yarn around the pencil within that section, not too tightly but not too loosely either. Then count how many wraps you’ve made. There’s a chart where you can look up exactly what yarn weight you have on hand....
For DK yarn, for example, that’s 11 wpi, 22 stitches per 10 cm....
Sock yarn, on the other hand, is 14 wpi, 28 stitches per 10 cm
And so on.... You can find the corresponding chart and also videos showing how to do the wrapping online.

Steffi has already explained this quite well. Since I spin a lot (on a spinning wheel, of course), WPI is the easiest and fastest way to determine what needle size I can use to work with the yarn.
Unfortunately, this measurement hasn’t really caught on in German-speaking countries yet. In my opinion, it would make it easier to compare a wide variety of yarns.