東南アジア大陸部言語連合

The Mainland Southeast Asia linguistic area is one of the most dramatic of linguistic areas in terms of the surface similarity of the languages involved, to the extent that early linguists tended to group them all into a single family, although the modern consensus places them into numerous unrelated families. The area stretches from Thailand to China and is home to speakers of languages of the Sino-Tibetan, Hmong–Mien (or Miao–Yao), Tai–Kadai, Austronesian (represented by Chamic) and Mon–Khmer families.

Neighbouring languages across these families, though presumed unrelated, often have similar features, which are believed to have spread by diffusion. A well-known example is the similar tone systems in Sinitic languages (Sino-Tibetan), Hmong–Mien, Tai languages (Kadai) and Vietnamese (Mon–Khmer). Most of these languages passed through an earlier stage with three tones on most syllables (but no tonal distinctions on checked syllables ending in a stop consonant), which was followed by a tone split where the distinction between voiced and voiceless consonants disappeared but in compensation the number of tones doubled. These parallels led to confusion over the classification of these languages, until André-Georges Haudricourt showed in 1954 that tone was not an invariant feature, by demonstrating that Vietnamese tones corresponded to certain final consonants in other languages of the Mon–Khmer family, and proposed that tone in the other languages had a similar origin.

Similarly, the unrelated Khmer (Mon–Khmer), Cham (Austronesian) and Lao (Kadai) languages have almost identical vowel systems. Many languages in the region are of the isolating (or analytic) type, with mostly monosyllabic morphemes and little use of inflection or affixes, though a number of Mon–Khmer languages have derivational morphology. Shared syntactic features include classifiers, object–verb order and topic–comment structure, though in each case there are exceptions in branches of one or more families.