Using it is a case of closing it ... and my Mitutoyo 530-122 Vernier caliper. The former two are my quick measurement tools of choice, when you see a product dimension in one of my Hackaday ...

I can use the vernier caliper for this again, as it has both metric and imperial measurements on it. I need to measure the diameter here, the distance across the entire bolt head. That's 13 ...

Of course, there’s also the old machinist’s trick (last item) about zeroing out the calipers after reading the diameter of one of the holes and then measuring the outside-to-outside distance ...