Medieval coins were stamped by a round, carved die, but they were defined in value by weight. This meant that sometimes a coin’s stamped round part was not sufficient to make up its value, so the coin had to include some metal outside the stamp. It was a perfect opportunity for a special kind of bank heist: coin shaving.
Copper and silver coins, which were the most common in daily circulation, could be shaved to match the stamped circle. They looked right, but their weight was no longer the correct value. Only a merchant or smith who had a very accurate balance for gold and silver could tell if a coin was still accurate or not.
Then again, supposedly accurate, official coins might have been made wrong. In that case, they could be shaved without changing the value of the coin. And the individual who did the shaving would make a profit at the government’s expense. Very accurate balances were controlled by goldsmith guilds; in medieval Germany, it was specifically illegal for a private person to own one.
Additionally, there was always the problem of purity of metals. Copper and silver could form alloys with tin or lead, while looking much the same. Tin and lead had their uses, but scarcity and value were not among their attributes.
Partly due to these two problems, use of coins in the medieval market was completely unlike ours today. First, we value old coins today; to them, old coins were much less valuable than new ones. Old coins were more likely to be shaved, after all. Newly-minted coins had some indication of the time they were struck; before the Anno Domini year counting system had become widespread, the king’s image placed the coin at least within his reign, and there was probably a number to indicate which year of his reign.
Second, today we take it for granted that a national currency is used within its country of origin. International medieval fairs were more like today’s black markets, where a buyer might offer German marks or US dollars, and the seller can choose which he’ll take. At a medieval fair, currency of all sorts circulated. Big fairs had money traders who managed exchange rates, which at first were just based on people’s preferences.
Strong central governments controlled currency; weak ones did not. In effect, a strong king like Charlemagne only permitted his appointed officials to mint coins, because he had the military power to punish anyone who minted coins without permission. Strong central governments wanted their money to be readily accepted at fairs, so they oversaw coin minting and made sure that pure metals and accurate weights were used.
During the medieval period, much or most of Europe did not have this type of strong government. Any small governing entity could strike coins, as long as it had some metal to make them with. Bishops and counts made coins, as did private moneyers. Every modern European nation, however small, was cut into even smaller portions during the Middle Ages. The Netherlands was divided into at least ten smaller parts, each of which minted coins—including dukes, counts and bishops.
There was a regular cycle of new coins that were shaved and worn down, then collected and remelted, then struck into a slightly newer design and released again. The newest coin from the strongest ruler was your best bet for lasting value. If you accepted a poor coin from a weak ruler, you might find yourself unable to give it away, let alone buy anything with it at a large fair. At a big fair, there might be a hundred competing coins in circulation.