Speaking Thai For Beginners

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Thai, Central Thai, or Siamese, is the national and official language of Thailand and the first language of the Thai people and the vast majority of Thai Chinese. It is a member of the Tai group of the Tai-Kadai language family. Over half of its words are borrowed from Pali, Sanskrit, Mon, and Old Khmer. It is a tonal and analytic language.

Thai also has a complex orthography and system of relational markers. Spoken Thai is mutually intelligible with Laotian, the language of Laos; the two languages are written with slightly different scripts but are linguistically similar.

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Dialects and related languages

Thai is the official language of Thailand, natively spoken by over 20 million people (2000). Standard Thai is based on the register of the educated classes of Bangkok. In addition to Central Thai, Thailand is home to other related Tai languages. Although linguists usually classify these idioms as related, but distinct languages, native speakers often identify them as regional variants or dialects of the "same" Thai language, or as "different kinds of Thai".

Related languages

• Isan (Northeastern Thai), the language of the Isan region of Thailand, a collective term for the various Lao dialects spoken in Thailand that show some Central Thai influences, which is written with the Thai script. It is spoken by about 20 million people. Thais from both inside and outside the Isan region often simply call this variant "Lao" when speaking informally.
• Northern Thai (Phasa Nuea, Lanna, Kam Mueang, or Thai Yuan), spoken by about 6 million (1983) in the formerly independent kingdom of Lanna (Chiang Mai). Shares strong similarities with Lao to the point that in the past the Siamese Thais referred to it as Lao.
• Southern Thai (Thai Tai, Pak Tai, or Dambro), spoken by about 4.5 million (2006)
• Phu Thai, spoken by about half a million around Nakhon Phanom Province, and 300,000 more in Laos and Vietnam (2006).
• Phuan, spoken by 200,000 in central Thailand and Isan, and 100,000 more in northern Laos (2006).
• Shan (Thai Luang, Tai Long, Thai Yai), spoken by about 100,000 in north-west Thailand along the border with the Shan States of Burma, and by 3.2 million in Burma (2006).
• Lü (Lue, Yong, Dai), spoken by about 1,000,000 in northern Thailand, and 600,000 more in Sipsong Panna, Burma, and Laos (1981-2000).
• Nyaw language, spoken by 50,000 in Nakhon Phanom Province, Sakhon Nakhon Province, Udon Thani Province of Northeast Thailand (1990).
• Song, spoken by about 30,000 in central and northern Thailand (2000).

Registers

Central Thai is composed of several distinct registers, forms for different social contexts:

• Street or Common Thai (???????, phasa phut, spoken Thai): informal, without polite terms of address, as used between close relatives and friends.
• Elegant or Formal Thai (?????????, phasa khian, written Thai): official and written version, includes respectful terms of address; used in simplified form in newspapers.
• Rhetorical Thai: used for public speaking.
• Religious Thai: (heavily influenced by Sanskrit and P?li) used when discussing Buddhism or addressing monks.
• Royal Thai (?????????, racha sap): influenced by Khmer, this is used when addressing members of the royal family or describing their activities. (See Monarchy of Thailand § Rachasap.)

Most Thais can speak and understand all of these contexts. Street and Elegant Thai are the basis of all conversations. Rhetorical, religious, and royal Thai are taught in schools as the national curriculum.

Speaking Thai For Beginners Video

Script

Many scholars believe that the Thai script is derived from the Khmer script, which is modeled after the Brahmic script from the Indic family. However, in appearance, Thai is closer to Thai Dam script, which may have the same Indian origins as the Khmer script. The language and its script are closely related to the Lao language and script. Most literate Lao are able to read and understand Thai, as more than half of the Thai vocabulary, grammar, intonation, vowels and so forth are common with the Lao language. Much like the Burmese adopted the Mon script (which also has Indic origins), the Thais adopted and modified the Khmer script to create their own writing system. While in Thai the pronunciation can largely be inferred from the script, the orthography is complex, with silent letters to preserve original spellings and many letters representing the same sound. While the oldest known inscription in the Khmer language dates from 611 CE, inscriptions in Thai writing began to appear around 1292 CE. Notable features include:

1. It is an abugida script, in which the implicit vowel is a short /a/ in a syllable without final consonant and a short /o/ in a syllable with final consonant.
2. Tone markers are placed above the final onset consonant of the syllable.
3. Vowels sounding after a consonant are nonsequential: they can be located before, after, above or below the consonant, or in a combination of these positions.

Transcription

There is no universally applied method for transcribing Thai into the Latin alphabet. For example, the name of the main airport is transcribed variously as Suvarnabhumi, Suwannaphum, or Suwunnapoom. Guide books, textbooks and dictionaries may each follow different systems. For this reason, most language courses recommend that learners master the Thai script.

Official standards are the Royal Thai General System of Transcription (RTGS), published by the Royal Institute of Thailand, and the almost identical ISO 11940-2 defined by the International Organization for Standardization. The RTGS system is increasingly used in Thailand by central and local governments, especially for road signs. Its main drawbacks are that it does not indicate tone or vowel length. As the system is based on pronunciation, not orthography, reconstruction of Thai spelling from RTGS romanisation is not possible.

Transliteration

The ISO published an international standard for the transliteration of Thai into Roman script in September 2003 (ISO 11940). By adding diacritics to the Latin letters, it makes the transcription reversible, making it a true transliteration. Notably, this system is used by Google Translate, although it seems not to appear in many other contexts, such as textbooks and other instructional media. This may be because the particular problems of writing Thai for foreigners, including silent letters and placement of vowel markers, decrease the usefulness of literal transliteration.

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Phonology

Consonants

Initials

Thai distinguishes three voice-onset times among plosive and affricate consonants:

• voiced
• tenuis (unvoiced, unaspirated)
• aspirated

Where English makes a distinction between voiced /b/ and aspirated /p?/, Thai distinguishes a third sound that is neither voiced nor aspirated, which occurs in English only as an allophone of /p?/, for example after an /s/ as in the sound of the p in "spin". There is similarly an alveolar /d/, /t/, /t?/ triplet in Thai. In the velar series there is a /k/, /k?/ pair and in the postalveolar series a /t??/, /t???/ pair, but the language lacks the corresponding voiced sounds /?/ and /d?/. (In loanwords from English, English /?/ and /d??/ are borrowed as the tenuis stops /k/ and /t??/.)

In each cell below, the first line indicates International Phonetic Alphabet (IPA), the second indicates the Thai characters in initial position (several letters appearing in the same box have identical pronunciation). Note also that ?, one of the two h letters, is also used to help write certain tones (described below).

Finals

Although the overall 44 Thai consonant letters provide 21 sounds in case of initials, the case for finals is different. For finals, only eight sounds, as well as no sound, called m?tr? (?????) are used. To demonstrate, at the end of a syllable, ? (/b/) and ? (/d/) are devoiced, becoming pronounced as /p/ and /t/ respectively. Additionally, all plosive sounds are unreleased. Hence, final /p/, /t/, and /k/ sounds are pronounced as [p?], [t?], and [k?] respectively.

Of the consonant letters, excluding the disused ? and ?, six (? ? ? ? ? ?) cannot be used as a final and the other 36 are grouped as following.

Clusters

In Thai, each syllable in a word is considered separate from the others, so combinations of consonants from adjacent syllables are never recognised as a cluster. Thai has phonotactical constraints that define permissible syllable structure, consonant clusters, and vowel sequences. Original Thai vocabulary introduces only 11 combined consonantal patterns:

• /kr/ (??), /kl/ (??), /kw/ (??)
• /k?r/ (??,??), /k?l/ (??,??), /k?w/ (??,??)
• /pr/ (??), /pl/ (??)
• /p?r/ (??), /p?l/ (??,??)
• /tr/ (??)

The number of clusters increases when a few more combinations are presented in loanwords such as /t?r/ (??) in ?????? (/int?ra:/, from Sanskrit indr?) or /fr/ (??) in ??? (/fri:/, from English free); however, it can be observed that Thai language supports only those in initial position, with either /r/, /l/, or /w/ as the second consonant sound and not more than two sounds at a time.

Vowels

The vowel nuclei of the Thai language, which includes monophthongs and opening diphthongs, are given in the following table. The top entry in every cell is the symbol from the International Phonetic Alphabet, the second entry gives the spelling in the Thai alphabet, where a dash (-) indicates the position of the initial consonant after which the vowel is pronounced. A second dash indicates that a final consonant must follow.

The vowels each exist in long-short pairs: these are distinct phonemes forming unrelated words in Thai, but usually transliterated the same: ??? (khao) means "he" or "she", while ??? (khao) means "white".

The long-short pairs are as follows:

There are also closing diphthongs and triphthongs in Thai, which Tingsabadh & Abramson (1993) analyze as underlyingly /Vj/ and /Vw/. For purposes of determining tone, those marked with an asterisk are sometimes classified as long:

Tones

There are five phonemic tones: mid, low, falling, high, and rising, sometimes referred to in older reference works as rectus, gravis, circumflexus, altus, and demissus, respectively. The table shows an example of both the phonemic tones and their phonetic realization, in the IPA.

The full complement of tones exists only in so-called "live syllables", those that end in a long vowel or a sonorant (/m/, /n/, /?/, /j/, /w/).

For "dead syllables", those that end in a plosive (/p/, /t/, /k/) or in a short vowel, only three tonal distinctions are possible: low, high, and falling. Because syllables analyzed as ending in a short vowel may have a final glottal stop (especially in slower speech), all "dead syllables" are phonetically checked, and have the reduced tonal inventory characteristic of checked syllables.

Unchecked syllables

Checked syllables

In some English loanwords, closed syllables with long vowel ending in an obstruent sound, have high tone, and closed syllables with short vowel ending in an obstruent sound have falling tone.

¹ May be /bá:s.kêt.b??l/ in educated speech.

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Grammar

From the perspective of linguistic typology, Thai can be considered to be an analytic language. The word order is subject-verb-object, although the subject is often omitted. Thai pronouns are selected according to the gender and relative status of speaker and audience.

Adjectives and adverbs

There is no morphological distinction between adverbs and adjectives. Many words can be used in either function. They follow the word they modify, which may be a noun, verb, or another adjective or adverb.

• ?????? (khon uan, [k?on ?ûan ]) a fat person
• ????????????? (khon thi uan reo, [khon t?î: ?ûan rew]) a person who became fat quickly

Comparatives take the form "A X ???? B" (kwa, [kwà:]), A is more X than B. The superlative is expressed as "A X ??????" (thi sut, [t?î:sùt]), A is most X.

• ?????????????? (khao uan kwa chan, [k??w ?ûan kwà: t????n]) S/he is fatter than me.
• ????????????? (khao uan thi sut, [k??w ?ûan t?î:sùt]) S/he is the fattest (of all).

Because adjectives can be used as complete predicates, many words used to indicate tense in verbs (see Verbs:Tense below) may be used to describe adjectives.

• ?????? (chan hiu, [t????n h?w]) I am hungry.
• ???????? (chan cha hiu, [t????n t??à? h?w]) I will be hungry.
• ??????????? (chan kamlang hiu, [t????n kamla? h?w]) I am hungry right now.
• ?????????? (chan hiu laeo, [t????n h?w l??:w]) I am already hungry.

Verbs

Verbs do not inflect. They do not change with person, tense, voice, mood, or number; nor are there any participles.

• ???????? (chan ti khao, [t????n ti: k??w]), I hit him.
• ???????? (khao ti chan, [k??w ti: t????n]), He hit me.

The passive voice is indicated by the insertion of ??? (thuk, [t?ù:k]) before the verb. For example:

• ???????? (khao thuk ti, [k??w t?ù:k ti:]), He is hit. This describes an action that is out of the receiver's control and, thus, conveys suffering.

To convey the opposite sense, a sense of having an opportunity arrive, ??? (dai, [dâj], can) is used. For example:

• ???????????????????????? (khao cha dai pai thiao mueang lao, [k??w t??a? dâj paj t?îow m?:a? la:w]), He gets to visit Laos.

Note, dai ([dâj] and [dâ:j]), though both spelled ???, convey two separate meanings. The short vowel dai ([dâj]) conveys an opportunity has arisen and is placed before the verb. The long vowel dai ([dâ:j]) is placed after the verb and conveys the idea that one has been given permission or one has the ability to do something. Also see the past tense below.

• ???????? (khao ti dai, [k??w ti: dâ:j]), He is/was allowed to hit or He is/was able to hit

Negation is indicated by placing ??? (mai,[mâj] not) before the verb.

• ????????, (khao mai ti) He is not hitting. or He doesn't hit.

Tense is conveyed by tense markers before or after the verb.

Tense markers are not required.

Thai exhibits serial verb constructions, where verbs are strung together. Some word combinations are common and may be considered set phrases.

• ???????????? (khao pai kin khao, [k??w paj kin k?â:w]) He went out to eat, literally He go eat rice
• ??????????????? (chan fang mai khao chai, [t????n fa? mâj k?âw t??aj]) I don't understand what was said, literally I listen not understand
• ?????? (khao ma, [k?âw ma:]) Come in, literally enter come
• ?????! (ok pai, [???:k paj]) Leave! or Get out!, literally exit go

Nouns

Nouns are uninflected and have no gender; there are no articles.

Nouns are neither singular nor plural. Some specific nouns are reduplicated to form collectives: ???? (dek, child) is often repeated as ???? ? (dek dek) to refer to a group of children. The word ??? (phuak, [p?ûak]) may be used as a prefix of a noun or pronoun as a collective to pluralize or emphasise the following word. (?????, phuak phom, [p?ûak p??m], we, masculine; ?????? phuak rao, [p?ûak raw], emphasised we; ?????? phuak ma, (the) dogs). Plurals are expressed by adding classifiers, used as measure words (????????), in the form of noun-number-classifier (????????, "teacher five person" for "five teachers"). While in English, such classifiers are usually absent ("four chairs") or optional ("two bottles of beer" or "two beers"), a classifier is almost always used in Thai (hence "chair four item" and "beer two bottle").

Pronouns

Subject pronouns are often omitted, with nicknames used where English would use a pronoun. See informal and formal names for more details. Pronouns, when used, are ranked in honorific registers, and may also make a T-V distinction in relation to kinship and social status. Specialised pronouns are used for those with royal and noble titles, and for clergy. The following are appropriate for conversational use:

The reflexive pronoun is ?????? (tua eng), which can mean any of: myself, yourself, ourselves, himself, herself, themselves. This can be mixed with another pronoun to create an intensive pronoun, such as ???????? (tua phom eng, lit: I myself) or ????????? (tua khun eng, lit: you yourself). Thai also does not have a separate possessive pronoun. Instead, possession is indicated by the particle ??? (khong). For example, "my mother" is ???????? (mae khong phom, lit: mother of I). This particle is often implicit, so the phrase is shortened to ????? (mae phom). Plural pronouns can be easily constructed by adding the word ??? (puak) in front of a singular pronoun as in ?????? (puak khao) meaning they or ?????? (puak thoe) meaning the plural sense of you. The only exception to this is ??? (rao), which can be used as singular (informal) or plural, but can also be used in the form of ?????? (puak rao), which is only plural.

Thai has many more pronouns than those listed above. Their usage is full of nuances. For example:

• "?? ??? ??? ????? ??? ?? ??? ????? ???????? ???????? ????? ??? ??????? ?????????????? ???? ???" all translate to "I", but each expresses a different gender, age, politeness, status, or relationship between speaker and listener.
• ??? (rao) can be first person (I), second person (you), or both (we), depending on the context.
• Children or younger female could use or being referred by word ??? (nu) when talking with older person. The word ??? could be both feminine first person (I) and feminine second person (you) and also neuter first and neuter second person for children.
  • ??? commonly means rat or mouse. It could refers to small creature in general
• The second person pronoun ??? (thoe) (lit: you) is semi-feminine. It is used only when the speaker or the listener (or both) are female. Males usually don't address each other by this pronoun.
• Both ??? (khun) and ??? (thoe) are polite neuter second person pronouns. However, ?????? (khun thoe) is a feminine derogative third person.
• Instead of a second person pronoun such as "???" (you), it is much more common for unrelated strangers to call each other "??? ???? ??? ??? ??? ?? ?? ???" (brother/sister/aunt/uncle/granny).
• To express deference, the second person pronoun is sometimes replaced by a profession, similar to how, in English, presiding judges are always addressed as "your honor" rather than "you". In Thai, students always address their teachers by "???" or "??????" or "???????" (each means "teacher") rather than ??? (you). Teachers, monks, and doctors are almost always addressed this way.

Particles

The particles are often untranslatable words added to the end of a sentence to indicate respect, a request, encouragement or other moods (similar to the use of intonation in English), as well as varying the level of formality. They are not used in elegant (written) Thai. The most common particles indicating respect are ???? (khrap, [k?ráp], with a high tone) when the speaker is male, and ??? (kha, [k?â], with a falling tone) when the speaker is female; these can also be used to indicate an affirmative, though the ??? (falling tone) is changed to a ?? (high tone).

Other common particles are:

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Register

As noted above, Thai has several registers, each having certain usages, such as colloquial, formal, literary, and poetic. Thus, the word "eat" can be ??? (kin; common), ??? (daek; vulgar), ??? (yat; vulgar), ?????? (boriphok; formal), ????????? (rapprathan; formal), ??? (chan; religious), or ???? (sawoei; royal), as illustrated below:

Thailand also uses the distinctive Thai six-hour clock in addition to the 24-hour clock.

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Vocabulary

Other than compound words and words of foreign origin, most words are monosyllabic.

Chinese-language influence was strong until the 13th century when the use of Chinese characters was abandoned, and replaced by Sanskrit and Pali scripts. However, the vocabulary of Thai retains many words borrowed from Middle Chinese.

Later most vocabulary was borrowed from Sanskrit and P?li; Buddhist terminology is particularly indebted to these. Indic words have a more formal register, and may be compared to Latin and French borrowings in English. Old Khmer has also contributed its share, especially in regard to royal court terminology. Since the beginning of the 20th century, however, the English language has had the greatest influence, especially for scientific, technical, international, and other modern terms. Many Teochew Chinese words are also used, some replacing existing Thai words (for example, the names of basic numbers; see also Sino-Xenic).

Arabic-origin

Chinese-origin

From Middle Chinese or Teochew Chinese.

English-origin

French-origin

Khmer-origin

From Old Khmer.

Portuguese-origin

The Portuguese were the first Western-nation to arrive in what is modern-day Thailand in the 16th century during the Ayutthaya period. Its influence in trade, especially weaponry, allowed them to establish a community just outside the capital and practice their faith, as well as exposing and converting the locals to Christianity. Thus, Portuguese words involving trade and religion were introduced and used by the locals.

History

Thai has undergone various historical sound changes. Some of the most significant changes, at least in terms of consonants and tones, occurred between Old Thai spoken when the language was first written and Thai of present, reflected in the orthography.

Old Thai

Old Thai had a three-way tone distinction on live syllables (those not ending in a stop), with no possible distinction on dead syllables (those ending in a stop, i.e. either /p/, /t/, /k/ or the glottal stop which automatically closes syllables otherwise ending in a short vowel).

There was a two-way voiced vs. voiceless distinction among all fricative and sonorant consonants, and up to a four-way distinction among stops and affricates. The maximal four-way occurred in labials (/p p? b ?b/) and dentals (/t t? d ?d/); the three-way distinction among velars (/k k? ?/) and palatals (/t? t?? d?/), with the glottalized member of each set apparently missing.

The major change between old and modern Thai was due to voicing distinction losses and the concomitant tone split. This may have happened between about 1300 and 1600 AD, possibly occurring at different times in different parts of the Thai-speaking area. All voiced-voiceless pairs of consonants lost the voicing distinction:

• Plain voiced stops (/b d ? d?/) became voiceless aspirated stops (/p? t? k? t??/).
• Voiced fricatives became voiceless.
• Voiceless sonorants became voiced.

However, in the process of these mergers the former distinction of voice was transferred into a new set of tonal distinctions. In essence, every tone in Old Thai split into two new tones, with a lower-pitched tone corresponding to a syllable that formerly began with a voiced consonant, and a higher-pitched tone corresponding to a syllable that formerly began with a voiceless consonant (including glottalized stops). An additional complication is that formerly voiceless unaspirated stops/affricates (original /p t k t? ?b ?d/) also caused original tone 1 to lower, but had no such effect on original tones 2 or 3.

The above consonant mergers and tone splits account for the complex relationship between spelling and sound in modern Thai. Modern "low"-class consonants were voiced in Old Thai, and the terminology "low" reflects the lower tone variants that resulted. Modern "mid"-class consonants were voiceless unaspirated stops or affricates in Old Thai--precisely the class that triggered lowering in original tone 1 but not tones 2 or 3. Modern "high"-class consonants were the remaining voiceless consonants in Old Thai (voiceless fricatives, voiceless sonorants, voiceless aspirated stops). The three most common tone "marks" (the lack of any tone mark, as well as the two marks termed mai ek and mai tho) represent the three tones of Old Thai, and the complex relationship between tone mark and actual tone is due to the various tonal changes since then. Note also that since the tone split, the tones have changed in actual representation to the point that the former relationship between lower and higher tonal variants has been completely obscured. Furthermore, the six tones that resulted after the three tones of Old Thai were split have since merged into five in standard Thai, with the lower variant of former tone 2 merging with the higher variant of former tone 3, becoming the modern "falling" tone.

Early Old Thai also apparently had velar fricatives /x ?/ as distinct phonemes. These were represented by the now-obsolete letters ? kho khuat and ? kho khon, respectively. During the Old Thai period, these sounds merged into the corresponding stops /k? ?/, and as a result the use of these letters became unstable.

At some point in the history of Thai, a palatal nasal phoneme /?/ also existed, inherited from Proto-Tai. A letter ? yo ying also exists, which is used to represent a palatal nasal in words borrowed from Sanskrit and Pali, and is currently pronounced /j/ at the beginning of a syllable but /n/ at the end of a syllable. Most native Thai words that are reconstructed as beginning with /?/ are also pronounced /j/ in modern Thai, but generally spelled with ? yo yak, which consistently represents /j/. This suggests that /?/ > /j/ in native words occurred in the pre-literary period. It is unclear whether Sanskrit and Pali words beginning with /?/ were borrowed directly with a /j/, or whether a /?/ was re-introduced, followed by a second change /?/ > /j/.

Proto-Tai also had a glottalized palatal sound, reconstructed as /?j/ in Li Fang-Kuei (1977). Corresponding Thai words are generally spelled ??, which implies an Old Thai pronunciation of /hj/ (or /j?/), but a few such words are spelled ??, which implies a pronunciation of /?j/ and suggests that the glottalization may have persisted through to the early literary period.

Vowel developments

The vowel system of modern Thai contains nine pure vowels and three centering diphthongs, each of which can occur short or long. According to Li (1977), however, many Thai dialects have only one such short-long pair (/a a:/), and in general it is difficult or impossible to find minimal short-long pairs in Thai that involve vowels other than /a/ and where both members have frequent correspondences throughout the Tai languages. More specifically, he notes the following facts about Thai:

• In open syllables, only long vowels occur. (This assumes that all apparent cases of short open syllables are better described as ending in a glottal stop. This makes sense from the lack of tonal distinctions in such syllables, and the glottal stop is also reconstructible across the Tai languages.)
• In closed syllables, the long high vowels /i: ?: u:/ are rare, and cases that do exist typically have diphthongs in other Tai languages.
• In closed syllables, both short and long mid /e e: o o:/ and low /? ?: ? ?:/ do occur. However, generally, only words with short /e o/ and long /?: ?:/ are reconstructible back to Proto-Tai.
• Both of the mid back unrounded vowels /? ?:/ are rare, and words with such sounds generally cannot be reconstructed back to Proto-Tai.

Furthermore, the vowel that corresponds to short Thai /a/ has a different and often higher quality in many of the Tai languages compared with the vowel corresponding to Thai /a:/.

This leads Li to posit the following:

1. Proto-Tai had a system of nine pure vowels with no length distinction, and possessing approximately the same qualities as in modern Thai: high /i ? u/, mid /e ? o/, low /? a ?/.
2. All Proto-Tai vowels were lengthened in open syllables, and low vowels were also lengthened in closed syllables.
3. Modern Thai largely preserved the original lengths and qualities, but lowered /?/ to /a/, which became short /a/ in closed syllables and created a phonemic length distinction /a a:/. Eventually, length in all other vowels became phonemic as well and a new /?/ (both short and long) was introduced, through a combination of borrowing and sound change. Li believes that the development of long /i: ?: u:/ from diphthongs, and the lowering of /?/ to /a/ to create a length distinction /a a:/, had occurred by the time of Proto-Southwestern-Tai, but the other missing modern Thai vowels had not yet developed.

Note that not all researchers agree with Li. Pittayaporn (2009), for example, reconstructs a similar system for Proto-Southwestern-Tai, but believes that there was also a mid back unrounded vowel /?/ (which he describes as /?/), occurring only before final velar /k ?/. He also seems to believe that the Proto-Southwestern-Tai vowel length distinctions can be reconstructed back to similar distinctions in Proto-Tai.

Source of the article : Wikipedia