The Sino-Tibetan languages, in a few sources also known as Trans-Himalayan, are a family of more than 400 languages spoken in East Asia, Southeast Asia and South Asia. The family is second only to Indo-European in terms of the number of native speakers. The Sino-Tibetan languages with the most native speakers are the varieties of Chinese (1.3 billion speakers), Burmese (33 million), and the Tibetic languages (6 million), but many Sino-Tibetan languages are spoken by small communities in remote mountain areas and as such are poorly documented. Unlike Western linguists, Chinese linguists generally include Tai-Kadai and Hmong-Mien languages within Sino-Tibetan.
Several low-level subgroups have been securely reconstructed, but reconstruction of a proto-language for the family as a whole is still at an early stage, so the higher-level structure of Sino-Tibetan remains unclear. Although the family is traditionally presented as divided into Sinitic (i.e. Chinese) and Tibeto-Burman branches, a common origin of the non-Sinitic languages has never been demonstrated. Several links to other language families have been proposed, but none has broad acceptance.
A genetic relationship between Chinese, Tibetan, Burmese and other languages was first proposed in the early 19th century and is now broadly accepted. The initial focus on languages of civilizations with long literary traditions has been broadened to include less widely spoken languages, some of which have only recently, or never, been written. However, the reconstruction of the family is much less developed than for families such as Indo-European or Austroasiatic. Difficulties have included the great diversity of the languages, the lack of inflection in many of them, and the effects of language contact. In addition, many of the smaller languages are spoken in mountainous areas that are difficult to access, and are often also sensitive border zones.
During the 18th century, several scholars had noticed parallels between Tibetan and Burmese, both languages with extensive literary traditions. Early in the following century, Brian Houghton Hodgson and others noted that many non-literary languages of the highlands of northeast India and Southeast Asia were also related to these. The name "Tibeto-Burman" was first applied to this group in 1856 by James Richardson Logan, who added Karen in 1858. The third volume of the Linguistic Survey of India, edited by Sten Konow, was devoted to the Tibeto-Burman languages of British India.
Studies of the "Indo-Chinese" languages of Southeast Asia from the mid-19th century by Logan and others revealed that they comprised four families: Tibeto-Burman, Tai, Mon–Khmer and Malayo-Polynesian. Julius Klaproth had noted in 1823 that Burmese, Tibetan and Chinese all shared common basic vocabulary but that Thai, Mon, and Vietnamese were quite different. Ernst Kuhn envisaged a group with two branches, Chinese-Siamese and Tibeto-Burman. August Conrady called this group Indo-Chinese in his influential 1896 classification, though he had doubts about Karen. Conrady's terminology was widely used, but there was uncertainty regarding his exclusion of Vietnamese. Franz Nikolaus Finck in 1909 placed Karen as a third branch of Chinese-Siamese.