My take on the traditional wpi tool.
WPI (Wraps Per Inch) is a traditional way of measuring the thickness of yarn. It allows you to decide if the yarn you have is suitable for a knitting pattern that's written for a different yarn.
Traditionally you just wrap your yarn around an inch gap, and count how many strands it takes to fill the gap. However, it's never a measurement I've had much luck with, how many strands you can fit in purely depends on how tightly you stretch the yarn, and how much you pack the yarn in. It works better with thinner yarns, but for fat fluffy yarns you almost might as well guess.
For the past few years I've been using a slightly different method to work out wpi. Instead of wrapping you just hold your yarn over the top of a line of a specified thickness. So for a wpi of 8, the line is 1/8th of an inch wide, if you have a reference card with a variety of thicknesses of lines you can just compare your yarn to a variety of lines and find the best match.
No wrapping, no packing, and I seem to get far more consistent results.
This little tool has a set of lines engraved on to it, covering a wide variety of yarn thicknesses, and a handy reminder of the range of wpi measurements that match the commercial names used for yarn thicknesses.
To use, just hold your strand over the gauge and find the nearest comparable block. It really is as simple as that. I hang mine from my wheel and use it to check my singles on a regular basis as I spin, to help me make my yarn more consistent. There's even a little hole to allow you to attach it to your wheel.
There's also a small groove at the top, measuring 1 inch, should you also want to measure in the more traditional way.
All current stocks are made from cherry veneered wood, acrylic plastic shown in alternative photos no long available.