There is no official reserve currency.. if two states trade a lot on bilateral basis .. they hold each other's currency ..and settle in local currencies .. Dollar is useful cuz most nations trade with US and have surplus dollars available because of trade deficits !
Discussion
I knew I'd regret that illustration point. I thought of a better one after, that gold is the standard because it has the right level of rarity. Rhodium is too rare and copper is too common etc.
There's no real point editing your posts here though because you never know which version a relay will pick up anyway. That's kind of a weakness and a strength, as well as taking me even further from my original point :)