The history of Korean currency dates back around 3rd century B.C., when knife coins, known as "Myeongdojun" circulated in the state of Yan and Gojoseon. The first iron and bronze coins were minted in Korea during the 15th year of the reign of King Seonjeong (996 A.D.). During the reign of King Sukjong, 1097 - 1107, a monetary system with a variety of cast coins was established. These coins included the Dongguk (Eastern Country), Haedong (Eastern Sea) and Samhan (Three States) coin series. Coins cast in copper and silver vase-shaped coins (unbyŏng) were issued in the 10th and 11th century but their circulation was limited. It was not until the beginning of the Joseon Dynasty (founded in 1392) that copper coins were minted for wide circulation. Soon after, however, Jeohwa, the first legal paper money, which was made from mulberry-bark, replaced metal coinage. Coins would not be issued again until early in the 16th century. Throughout this time imported Chinese coins also circulated, and grain and linen were used as commodity currencies. In the 17th century, coinage finally became the primary medium of exchange and twenty-four mints were established across Korea. From 1633 until 1892, coins denominated in mun and bearing the inscription Sang Pyeong Tong Bo were the most widely circulated currency.