The Thai language is spoken primarily in the central plains of Thailand and in the area around the Thai capital, Bangkok. From the 12th until the 20th century, Thailand was called Siam and its language was known as Siamese. In 1939 the country became the Kingdom of Thailand. Estimates of total numbers of Thai speakers varies greatly. On the lower end of the scale are estimates putting the number at around 20 million, or about 50 % of Thailand's total population while estimates on the high end put the figure at 37 million representing about 80 percent of the national population. This includes some 5 million Thai speaking ethnic Chinese and perhaps a half million speakers of Thai's Khorat dialect. Small populations of Thai speakers also live in Singapore, the United Arab Emirates, and the United States. The oldest known evidence of Thai literature is a Buddhist cosmography that dates to the 14th century.