Has become a tradition in the military. These coins vary in size. Normally about 38-40mm, the two shown here are 38.3mm for the CWO-2 coin (my rank) and 49.5mm for the new USS New Hampshire. The idea is, if you walk into an O club (Officers Club) and someone challenges you to show your coin and you don't have it, you buy the drinks. These are two of my favorites.