A WPI control card is used by spinners to calibrate the yarn thickness and thus facilitate even spinning, and it can then be used to grade the thickness of the finished spun yarn.
It can also be used to check the thickness of a yarn you already have in your stash. On the back there are graduations for the angle of the twisted yarn and the difference between S and Z twist. There is also a practical hole if you want to hang it on your spinning wheel.
3D printed in PLA filament.
PLA (polylactic acid or polylactide) is a bioplastic that is mainly made from starch from plants, such as sugar beets, sugar cane and corn.
