It is a rolling ruler in metric 20cm (item No. EA-304M) for measuring any line, particularly curves. It is super easy to use and the best part is that it is fast and accurate. Basically, this small white plastic wheel has markings all around it's circumference from 0 - 20 cm, with fractional increments at every mm too. It's a revolving, rotary measuring tool that allows you to easily and pretty accurately measure around curves and bends - so perfect when you need to check the size of armholes (armscyes) and necklines etc.
One one side it's labelled 0-80 divisions for ¼ scale. The wheel has dents all around so as to avoid slippery!
This rolling ruler also comes with imperial unit with circumference from 0 - 4in with fractional increments at every 1/8" too (both sides) under item No. EA-304E. When ordering, please select "unit" - metric or imperial (inch).