Tibetan is spoken by Tibetan people across a wide area of eastern Central Asia. Its classical written form is a major regional literary language, particulary in its use as in Buddhist writings. Tibetan is typically classified as a Tibeto-Burman language which in turn is, according to the most widespread theory, a branch of the Sino-Tibetan language family. Spoken Tibetan includes numerous regional varieties which, in many cases, are not mutually intelligible. Moreover, the boundaries between Tibetan and certain other Himalayan languages are sometimes unclear. In general, the dialects of central Tibet (including Lhasa), Kham, Amdo, and some smaller nearby areas are considered Tibetan dialects, while other forms, particularly Dzongkha, Sikkimese, Sherpa, and Ladakhi, are considered closely-related but separate languages. By this definition, Tibetan is spoken by approximately 6 million people across the Tibetan Plateau as well as by approximately 150,000 exile speakers in India and other countries.