In the ancient Greek world groups of smaller cities that had common trading interests and were too small to support individual mints formed monetary unions. These unions minted coins that were legal tender in all the member cities, which also split the profits from the coinage. These union coins might be struck in one federal mint, or several member cities might operate mints that shared in minting the union coinage. Often one side of these union coins bore a common symbol of the monetary union and the reverse side bore a symbol of the city where the coin was issued. Coins issued at different mints under the authority of a monetary union possessed a uniform weight and purity. These coins served as an international currency in their trading area, restricting the need for moneychangers, and avoiding the business hazards of fluctuations in foreign currency markets.
A monetary convention between Phocea and Mitylene, adopted around 400 b.c., represents one of the few agreements for a monetary union whose details survived for our inspection. The prerogative for striking coins alternated between each city on an annual basis. Each year one city closed its mint while the mint in the other city met the need for coins that year. These cities agreed to issue electrum coins identical in weight and purity, but without a uniform emblem, and the coins from either mint were legal tender in both cities.
At the end of the fifth century a group of towns and tribes on the western coast of Greece formed a monetary union, but knowledge of this union is limited to what can be gathered from the coins. Its coins were based on the silver stater of Corinth, a popular coin in the trading area around Italy and Sicily, and its coins bore identical images; a head of Athena on the obverse side, and Pegasus on the reverse side. This union thrived until the second century b.c. when the Roman invasion put an end to it.
The cities of Boeotia, north of Attica, joined into a monetary union that lasted from early times until the coming of Rome. The union began coining silver and later added a uniform bronze coinage. Although struck at different mints, the coins were uniform in weight and purity, and bore on the obverse side a badge that represented the union and on the reverse side a symbol of the city of origin.
One of the largest of the monetary unions federated as many as 43 cities at one time, including Corinth, Argos, and Lacedaemon. Its silver coins dominated trade within the Peloponnese from 280 b.c. to 146 b.c. when the area became a Roman province.
These unions were sometimes the by-products of military alliances. Mobilization of an army always created a demand for coinage, and armies were instrumental in spreading coinage. Also, cities that patronized the same religious temples, or sacred games and festivals, were more likely to issue a federated currency that supported shared activities.
The strengths of a monetary union remain very real in areas such as Europe, where small but sophisticated economies function in close geographical proximity. In 1998 the European Monetary Union planned the final step toward monetary unification with the introduction of a single European currency.