For most of the medieval era, the silver penny was the basic currency in all places. Silver was the most common precious metal; gold was too uncommon until the late medieval, and copper was not precious enough. Silver was precious, but not too much so. Copper and bronze coins were also used, but their value was fixed to the silver penny.
During the early medieval period, Anglo-Saxon and Merovingian kingdoms had poor control over their currency. As described in past entries, weak central government permitted anyone with local power to collect silver coins (including Roman ones) and melt them down to strike new local coins. Charlemagne was the first Germanic ruler with enough power to reform and stabilize currency across a wide area. At his peak, he ruled (directly or through vassals) most of modern France, Benelux, Germany, Switzerland, and Italy.
Charlemagne stipulated that his penny would be called a denarius, obviously following Roman tradition. 240 of them would make a pound (which was probably a little different from the modern pound). Over time, the medieval French silver penny was called a denier (probably rhymes with veneer); it remained the mainstay of the marketplace for many centuries. Interestingly, there was a theoretical coin that was not, apparently, actually minted in his time: the solidus, worth 12 denarii, therefore 20 of them in that same pound. Charlemagne’s solidus was later copied as an Anglo-Norman coin, becoming the shilling in medieval Latin patois. That’s why there are 20 shillings in a pound.
Charlemagne’s pennies were struck with unusually good dies, perhaps because his wide region of rule permitted him to bring in Italian craftsmen who still had some of the old skill. He wanted to control all minting in Aachen, his capital, but the region was just too large. His deniers were minted in Milan, Pavia, Paris, Lyon, Rouen, Maastricht and other cities, in addition to Aachen.
During Charlemagne’s time, and that of his immediate successors, he was able to control the value and quality of the denier. With the break-up of his empire, of course minting again went out of control. The coins during this time were often quite debased with other metals or bad weighing. Even some abbots minted coins.