Unlike mainland Korea, which was ruled only indirectly by the Mongols, Jeju was placed under direct Yuan administration in the late thirteenth century. Significant numbers of Mongol soldiers migrated to the island during this period, and their language acted as a superstratum that may have accelerated local language change. Linguist Yang Changyong speculates that the formation of Jeju as a language independent of Korean was influenced by Mongol. By the fifteenth century, when the invention of Hangul permits a detailed understanding of Korean phonology for the first time, Seoul Korean and Jeju were already divergent; the Seoul prestige dialect of fifteenth-century Middle Korean disallowed the diphthong , but Jeju does not.