ISO 11940-2

ISO 11940-2 is an ISO standard for a simplified transcription of the Thai language into Latin characters.

The full standard ISO 11940-2:2007 includes pronunciation rules and conversion tables of Thai consonants and vowels. It is a sequel to ISO 11940, describing a way to transform its transliteration into a broad transcription.


The standard ISO 11940 (to be renamed 11940-1) defines a strict and reversible transliteration of Thai orthography into Latin characters, by means of a host of diacritics. The result bears no resemblance to Thai pronunciation. The additional standard ISO 11940-2 describes a set of rules to transform the transliteration resulting from ISO 11940 based on Thai orthography into a broad transcription based on pronunciation, using only unadorned Latin letters. All information on vowel length and syllable tone is dropped, as well as the distinction between IPA /o/ and /ɔ/.

The standard explicitly mentions that whenever the full pronunciation of each word is necessary or needed, conversion of long vowels can be devised and tone rules can be added to the system to achieve the full pronunciation of each word. However no rules are included how to achieve this.


Although the standard is described as a procedure acting on the Thai orthography, the system is based on the pronunciation. Its rules can therefore be also described in terms of Thai phonology. Prominent features of ISO 11940-2 include:

  • uses only unmodified letters from the Latin alphabet; no diacritics
  • spells all vowels and diphthongs using only vowel letters: ⟨a⟩, ⟨e⟩, ⟨i⟩, ⟨o⟩, ⟨u⟩
    • single letters ⟨a⟩, ⟨e⟩, ⟨i⟩, ⟨o⟩, ⟨u⟩ are simple vowels with the same value as in the International Phonetic Alphabet (IPA)
    • digraphs with trailing ⟨e⟩ are simple vowels, ⟨ae⟩, ⟨oe⟩, ⟨ue⟩ sound like /ɛ, ɤ, ɯ/ respectively (and are perhaps chosen for their similarity to IPA ligatures: /æ, œ, ɯ/)
    • digraphs with trailing ⟨a⟩, ⟨i⟩, ⟨o⟩ are diphthongs, indicated by /a, j, w/ respectively in IPA
  • uses consonants as in IPA, except:
    • uses ⟨c⟩ for /tɕ/
    • digraphs with ⟨h⟩ (⟨ph⟩, ⟨th⟩, ⟨kh⟩, ⟨ch⟩) are aspirated (/pʰ, tʰ, kʰ, tɕʰ/) consonants, to distinguish them from the separate unaspirated ⟨p⟩, ⟨t⟩, ⟨k⟩, ⟨c⟩ (/p, t, k, tɕ/)
    • uses ⟨ng⟩ for /ŋ/, as in English
    • uses ⟨y⟩ for /j/, as in English
    • uses ⟨'⟩ for a glottal stop /ʔ/, as occurs when a syllable starts with a vowel

Transcription is according to pronunciation, not Thai orthography, especially notable in final consonants. Vowels are transcribed in sequence as pronounced, not as written in Thai script. Implied vowels, which are not written in Thai script, are inserted as pronounced. Written silent letters are omitted.


The result of applying the rules described in the standard is almost identical to the transcription defined by the Royal Thai General System of Transcription. One exception is preceding a syllable initial vowel by ⟨'⟩, representing the Thai null consonant อ, obviating the need to insert a dash in some words to preserve syllable boundaries. The other exception is the retention of the aspiration characteristic of the alveolo-palatal affricate. So while Thai ฉ, ช, and ฌ, are represented by ⟨ch⟩ as in RTGS, the Thai letter จ is written as ⟨c⟩.




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). The third line shows the ISO 11940-2 rendering.

Labial Alveolar Palatal Velar Glottal
Nasal [m]


Stop tenuis [p]




aspirated [pʰ]
voiced [b]

Fricative [f]
Approximant [w]

Trill [r]



Of the consonant letters, excluding the disused ฃ and ฅ, six (ฉ ผ ฝ ห อ ฮ) cannot be used as a final and the other 36 collapse into a very small repertoire of possible final consonant sounds and corresponding Latin letters. The consonants ย and ว when used as finals, form diphthongs and triphthongs with the preceding vowel, and ISO 11940-2 uses the vowel letters i and o in such cases.

Labial Alveolar Palatal Velar
Nasal [m]


Stop [p]
Approximant [w]




The basic vowels of the Thai language, from front to back and close to open, 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 third line contains the ISO 11940 symbol used.

  Front Back
unrounded unrounded rounded
short long short long short long
Close /i/
i ue u
Close-mid /e/
e oe o
Open-mid /ɛ/
ae o
Open     /a/
-ะ, -ั-

Thai vowels come in long-short pairs, forming distinct phonemes, but ISO 11940-2 represents both by the same symbol. Also the two phonemes IPA o and ɔ share a single Latin letter o.

The basic vowels can be combined into diphthongs and triphthongs.

Long Short ISO
Thai IPA Thai IPA
–าว /aːw/ เ–า /aw/ ao
เ–ว /eːw/ เ–็ว /ew/ eo
แ–ว /ɛːw/ aeo
–ิว /iw/ io
เ–ียว /iaw/ iao
เ–ีย /iːa/ เ–ียะ /ia/ ia
–ัว /uːa/ –ัวะ /ua/ ua
เ–ือ /ɯːa/ เ–ือะ /ɯa/ uea
–าย /aːj/ ไ–*, ใ–*, ไ–ย, -ัย /aj/ ai
–อย /ɔːj/ oi
โ–ย /oːj/
–ูย /uːj/ –ุย /uj/ ui
เ–ย /ɤːj/ oei
–วย /uaj/ uai
เ–ือย /ɯaj/ ueai

External links

ISO 11940

ISO 11940 is an ISO standard for the transliteration of Thai characters, published in 1998 and updated in September 2003 and confirmed in 2008. An extension to this standard named ISO 11940-2 defines a simplified transcription based on it.

List of ISO romanizations

List of ISO standards for transliterations and transcriptions (or romanizations):

ISO 9 — Cyrillic

ISO 233 — Arabic

ISO 259 — Hebrew

ISO 843 — Greek

ISO 3602 — Japanese (1989, last reviewed 2013)

ISO 7098 — Chinese

ISO 9984 — Georgian

ISO 9985 — Armenian

ISO 11940 — Thai

ISO 11940-2 — Thai (simplified)

ISO 11941 — Korean (different systems for North and South Korea – withdrawn in 2013)

ISO 15919 — Indic scripts

List of International Organization for Standardization standards, 11000-11999

This is a list of published International Organization for Standardization (ISO) standards and other deliverables. For a complete and up-to-date list of all the ISO standards, see the ISO catalogue.The standards are protected by copyright and most of them must be purchased. However, about 300 of the standards produced by ISO and IEC's Joint Technical Committee 1 (JTC1) have been made freely and publicly available.


Romanization or romanisation, in linguistics, is the conversion of writing from a different writing system to the Roman (Latin) script, or a system for doing so. Methods of romanization include transliteration, for representing written text, and transcription, for representing the spoken word, and combinations of both. Transcription methods can be subdivided into phonemic transcription, which records the phonemes or units of semantic meaning in speech, and more strict phonetic transcription, which records speech sounds with precision.

Romanization of Thai

There are many systems for the romanization of the Thai language, i.e. representing the language in Latin script. These include systems of transliteration, and transcription.

The most seen system in public space is Royal Thai General System of Transcription (RTGS)—the official scheme promulgated by the Royal Thai Institute. It is based on spoken Thai, but disregards tone, vowel length and a few minor sound distinctions.

The international standard ISO 11940 is a transliteration system, preserving all aspects of written Thai adding diacritics to the Roman


Its extension ISO 11940-2 defines a simplified transcription reflecting the spoken language. It is almost identical to RTGS.

Libraries in English-speaking countries use the ALA-LC Romanization.

In practice, often non-standard and inconsistent romanizations are used, especially for proper nouns and personal names. This is reflected, for example, in the name Suvarnabhumi Airport, which is spelled based on direct transliteration of the name's Sanskrit root.

Language learning books often use their own proprietary systems, none of which are used in Thai public space.

Royal Thai General System of Transcription

The Royal Thai General System of Transcription (RTGS) is the official system for rendering Thai words in the Latin alphabet. It was published by the Royal Institute of Thailand.It is used in road signs and government publications and is the closest method to a standard of transcription for Thai, but its use, by even the government, is inconsistent. The system is almost identical to the one that is defined by ISO 11940-2.

Thai language

Thai, Central Thai or Ayutthaya or Siamese (Thai: ภาษาไทย), is the sole official and national language of Thailand and the first language of the Central Thai people and vast majority of Thai of Chinese origin. It is a member of the Tai group of the Kra–Dai language family. Over half of Thai vocabulary is derived from or borrowed from Pali, Sanskrit, Mon and Old Khmer. It is a tonal and analytic language, similar to Chinese and Vietnamese.

Thai has a complex orthography and system of relational markers. Spoken Thai is mutually intelligible with Lao and Isan, fellow Southwestern Tai languages, to a significantly high degree where its speakers are able to effectively communicate each speaking their respective language. These languages are written with slightly different scripts but are linguistically similar and effectively form a dialect continuum.

Thai script

The Thai script (Thai: อักษรไทย; RTGS: akson thai; [ʔàksɔ̌ːn tʰāj] listen) is the abugida used to write Thai, Southern Thai and many other languages spoken in Thailand. The Thai alphabet itself (as used to write Thai) has 44 consonant symbols (Thai: พยัญชนะ, phayanchana), 15 vowel symbols (Thai: สระ, sara) that combine with 28 vowel symbols and four tone diacritics (Thai: วรรณยุกต์ or วรรณยุต, wannayuk or wannayut) to create characters mostly representing syllables.

Although commonly referred to as the "Thai alphabet", the script is in fact not a true alphabet but an abugida, a writing system in which the full characters represent consonants with diacritical marks for vowels; the absence of a vowel diacritic gives an implied 'a' or 'o'. Consonants are written horizontally from left to right, with vowels arranged above, below, to the left, or to the right of the corresponding consonant, or in a combination of positions.

Thai has its own set of Thai numerals that are based on the Hindu-Arabic numeral system (Thai: เลขไทย, lek thai), but the standard western Hindu-Arabic numerals (Thai: เลขฮินดูอารบิก, lek hindu arabik) are mainly used except for government documents and the license plates of military vehicles.

ISO standards by standard number

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