Phonemic tone is one of the most well-known of southeast Asian language characteristics. Many of the languages in the area have strikingly similar tone systems, which appear to have
developed in the same way.
Origin of tonal contrasts The tone systems of
Middle Chinese,
proto-Hmong–Mien,
proto-Tai and early Vietnamese all display a three-way tonal contrast in syllables lacking
stop endings. In traditional analyses, syllables ending in stops have been treated as a fourth or "
checked tone", because their distribution parallels that of syllables with nasal codas. Moreover, the earliest strata of loans display a regular correspondence between tonal categories in the different languages: The incidence of these tones in Chinese, Tai and Hmong–Mien words follows a similar ratio 2:1:1. Thus
rhyme dictionaries such as the
Qieyun divide the level tone between two volumes while covering each of the other tones in a single volume. Vietnamese has a different distribution, with tone B four times more common than tone C. It was long believed that tone was an invariant feature of languages, suggesting that these groups must be related. However this category cut across groups of languages with shared basic vocabulary. In 1954
André-Georges Haudricourt solved this paradox by demonstrating that Vietnamese tones corresponded to certain final consonants in other (atonal) Austroasiatic languages. He thus argued that the Austroasiatic proto-language had been atonal, and that its development in Vietnamese had been conditioned by these consonants, which had subsequently disappeared, a process now known as
tonogenesis. Haudricourt further proposed that tone in the other languages had a similar origin. Other scholars have since uncovered transcriptional and other evidence for these consonants in early forms of Chinese, and many linguists now believe that
Old Chinese was atonal. A smaller amount of similar evidence has been found for proto-Tai. Moreover, since the realization of tone categories as pitch contours varies so widely between languages, the correspondence observed in early loans suggests that the conditioning consonants were still present at the time of borrowing.
Loss of voicing with tone or register split A characteristic
sound change (a
phonemic split) occurred in most southeast Asian languages around 1000 AD. First, syllables with voiced initial consonants came to be pronounced with a lower pitch than those with unvoiced initials. In most of these languages, with a few exceptions such as
Wu Chinese, the voicing distinction subsequently disappeared, and the pitch contour became distinctive. In tonal languages, each of the tones split into two "registers", yielding a typical pattern of six tones in unchecked syllables and two in checked ones.
Pinghua and
Yue Chinese, as well as neighbouring Tai languages, have further tone splits in checked syllables, while many other Chinese varieties, including
Mandarin Chinese, have merged some tonal categories. Many non-tonal languages instead developed a register split, with voiced consonants producing
breathy-voiced vowels and unvoiced consonants producing
normally voiced vowels. Often, the breathy-voiced vowels subsequently went through additional, complex changes (e.g. diphthongization). Examples of languages affected this way are
Mon and
Khmer (Cambodian). Breathy voicing has since been lost in standard Khmer, although the vowel changes triggered by it still remain. Many of these languages have subsequently developed some voiced obstruents. The most common such sounds are and (often pronounced with some implosion), which result from former preglottalized and , which were common phonemes in many Asian languages and which behaved like voiceless obstruents. In addition, Vietnamese developed voiced fricatives through a different process (specifically, in words consisting of two syllables, with an initial, unstressed
minor syllable, the medial stop at the beginning of the stressed major syllable turned into a voiced fricative, and then the minor syllable was lost). ==Morphology and syntax==