This gauge has grooves for laying your strand of yarn to quickly determine your yarn's thickness.  Quickly and easily find your WPI.  Keep it handy to check your yarn when spinning to get technical data on your spinning.

There is also a one-inch notch at the sides to wrap yarns around and count your WPI the traditional way.

What is WPI?
WPI stands for "wraps per inch" and is one way to measure the thickness of a yarn.

Wrap your yarn around a gauge laying each strand next to each other, so that they don't overlap, for the width of one-inch.  Count the strands.  The number of strands of yarn required to fill one-inch is the yarn's wraps per inch.

WPI  tells you the thickness of the yarn, which is useful for designing yarn when spinning, checking a yarn's thickness when the label has disappeared or didn't include gauge information, or making substituting.