According to Wikipedia, there were a wide range of currencies in the Middle Ages:
The various Germanic states in the west all had coinages that imitated existing Roman and Byzantine forms. Gold continued to be minted until the end of the 7th century, when it was replaced by silver coins. The basic Frankish silver coin was the denarius or denier, while the Anglo-Saxon version was called a penny. From these areas, the denier or penny spread throughout Europe during the centuries from 700 to 1000. Copper or bronze coins were not struck, nor were gold except in Southern Europe. No silver coins denominated in multiple units were minted.
Say that someone had one of these forms of coinage (such a local German state's coinage, or some coins minted by a local ruler). That person then travels to a far off region and wants to spend their coins at a local inn, but the people have never seen those coins before. How would currency exchange be negotiated? Or would the coins in the new region be considered worthless?