Hohlpfennigs (brakteaty guziczkowe in Polish, duté mince in Czech) are an unusual group of coins. They are small, anonymous bracteates having their types surrounded with a massive convex ring. Small figures placed on them are much varied. There are plants, animals, humans, buildings and many more. This was a predominant form of coinage in Central and Northern Europe from the mid-thirteenth century to the mid-fourteenth century.

This characteristic form of coins resulted from the monetary system based on frequent (usually half-yearly) recoinage in Poland, Bohemia and eastern Germany. This practice was completely different than in other parts of Europe. What is more, the political disintegration of Poland significantly reduced the range of monetary emissions. This meant that the validity of money was significantly reduced both in time and in space. Therefore, the most important feature of the appearance of each coin was what distinguished it from other coins that were no longer valid. The multiplicity of types, sometimes very similar each other, combined with numerous emissions and frequent renovation (recoinage), makes it difficult for today's researcher to identify whether a given coin makes a different issue or just a die variation within one issue. This causes many problems related, among others, to the question of dating the hohlpfennigs and placing their production in a specific part of our country. German and Czech researchers also face similar issues.

There is no comprehensive study of this group of coins discovered in Polish hoards. Wars and border changes have meant that most of the old collections, both museum and private, have been dispersed. Nevertheless, large fragments of several most important Polish hoards discovered in the nineteenth or early twentieth century have survived (Brzegi, Wieleń or Radzanowo), but Silesian and Pomeranian collections were dispersed. Elements of these collections are rediscovered on the antiquarian market.

Thanks to the funding from the National Science Center, our research team is the first to try to explain the phenomenon of hohlpfennigs. The result of our work will provide a model to unravel similar issues in other countries.