For some reason, I had never bothered to pursue the relationship between string gauge and actual string diameter. However, when I sorted my stash of old strings after putting brand new Elixir medium strings in order to sound best at next week's concert, I found several strings for which I didn't know the gauge, meaning they would be next to useless in case of an emergency (which is typically the only situation under which I'd put on a used string). And I only had a metric vernier caliper available...