Throughout the world our economies depend on the concept of a medium of exchange. We have stock exchanges all over the globe and people buy and sell currencies from other countries, such as the US dollar, British pound, and Japanese yen. When you travel from country to country, you can typically walk into a bank and exchange one currency for another based on a standard of exchange that is current. This same concept was very much true in Biblical times as well.
Being a coin collector, and a theologian I found this article of particular interest. Great job!