Already under the Ptolemies, the coinage of Egypt circulated in a closed currency system: foreign money had to be exchanged for the local currency at the borders, and Egyptian currency remained in Egypt. This closed system continued intact under Roman rule until the end of the third century. The coins were “Alexandrian coins” after the city Alexandria, where they were minted. Two metals were used for coins in circulation in Egypt: billon, a silver alloy, was used for tetradrachms; and bronze for smaller denominations. Oversight of the coinage probably fell either to the idios logos, the highest financial official of Egypt, or to the dioiketes, head of the treasury in Alexandria. Since these provincial coins, with their great variety of types, are official documents of Roman rule, they are considered as excellent sources for study of the monetary, political, religious, artistic and cultural history of Greco-Roman Egypt.