i know it's a term commonly used in certain fields of commece, but like many others, it's not an 'honest' one!
It seems to me that you'd almost have to have several different metals or other substances, of widely varying values, for money. You need something that allows the wealthy man to carry reasonable wealth without needing a wagon train to carry it in, and something to allow the poor man to carry a coin without it being so small that it gets lost in the lint in his pocket. So, gold and copper, or some equivalent, and probably a couple of materials in between. ("Some equivalent" could be salt, or sugar, or spices, or jewels, or silk thread, or tulip bulbs, or, well, zillions of possibilities. Something widely valued and easily portable.) I suppose "have to" is an overstatement - you could assume that your setting has no easily traded materials of this kind, but that would change your society quite a bit; it would be something of a thought experiment. ChickenFreak
In fact, that tended to happen with currencies sooner or later. It's the historical equivalent of the government printing money to spend.
