of Tamna, which ruled the island until the twelfth century, may have spoken a Japonic language that left a substrate influence on Jeju. When exactly this putative Japonic language may have been replaced by the Koreanic ancestor of Jeju remains unclear. 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. Sixteenth- and seventeenth-century references to the language of Jeju by mainland Korean literati state that it was already unintelligible to mainland Koreans. Kim Sang-heon (1570―1652), who served as pacification commissioner ( ) on the island from 1601 to 1602, gives six words in the "provincial language" with clear cognates in modern Jeju and also writes: "The exiled man Shin Jangnyeong was originally a government interpreter. He said, 'The language of this island is most like Chinese, and the sounds they make while driving cattle and horses are yet more impossible to tell apart. Is this because the climate is not far from that of China, or because the Yuan dynasty once ruled and appointed officials here and the Chinese mingled with them?'... What is called the provincial language is but high and thin and cannot be understood." In 1629, the Korean government banned the emigration of Jeju Islanders to the mainland, further restricting linguistic contacts between Jeju and Korean. At the same time, the island was also used throughout the Joseon era (1392―1910) as a place of exile for disgraced scholar-officials. These highly educated speakers of Seoul Korean often tutored the children of their Jeju neighbors during their exile and established a continuous and significant Seoul Korean superstratum in Jeju. Jeju remained the dominant language of both private and public spheres under Japanese colonial rule (1910―1945), although many Japanese loanwords entered the lexicon, and many speakers were monolingual. Large-scale migration of Jeju people to Japan began in 1911,