The Byzantine Empire was the continuation of the Roman Empire in the Eastern Mediterranean, a fact that is also reflected in the gradual transition from Roman to Byzantine currency. Byzantine currency spread throughout most of the then known world and served as a model for coins minted by peoples in the medieval West and East. The solidus, the basic unit of the Byzantine monetary system, was the powerful gold coin of the Middle Ages, as it constituted a stable value for international transactions until the 11th century. The circulation of gold, silver and copper coins testifies to the high degree of monetization of the Byzantine economy. Important information about the organization of the Byzantine administrative system and the functioning of trade is provided by lead stamps and weights, respectively.