Thats an inside caliper/compass for measuring inside things. A ring would qualify I suppose LOL..........
Now if the points pointed in I have used those for lofting a pattern for the bow break water on that woodie bought I built. Its for getting crazy outside measurement where you can use a taped measure. Like around things..

Realist - Everybody in America is soft, and hates conflict. The cure for this, both in politics and social life, is the same -- hardihood. Give them raw truth.