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ISO 11940 is anISO standard for thetransliteration ofThai characters, published in 1998, updated in September 2003, and confirmed in 2008. A most notable usage of the standard is by Google Translate.An extension to this standard, namedISO 11940-2, defines a simplified transcription based on it.

Thetransliteration of the pureconsonants is derived from their usual pronunciation as an initial consonant. An unmarkedh is used to formdigraphs denotingaspirated consonants. High and low pairs of consonants are systematically differentiated by applying amacron to the high class consonant. Further differentiation of consonants with identicalphonetic function is obtained by leaving the most frequent unmarked, marking the second commonest by a dot below, marking the third commonest by a horn, and marking the fourth commonest by underlining. The use of a dot below has a similar effect to theIndological practice of distinguishingretroflex consonants by a dot below, but there are subtle differences – it is the transliterations of ธtho thong and ศso sala that are dotted below, not those of the correspondingretroflex consonants. The transliterations of consonants should be entered in the order base letter, macron if any, and then dot below, horn or "macron below".Only three consonants have the horn in their transliteration, ฅkho khon, ฒtho phuthao and ษso ruesi, and only one consonant has an underline, ฑtho nang montho.

The letterå is the only precomposed character specified in the output of transliteration.

Lakkhangyao (ๅ) has been shown only in combination with the vowel letters ฤ and ฦ. The standard simply lists ฤ and ฦ with the consonants andlakkhangyao with the vowels. An isolatedlakkhangyao would also be transliterated by a small letter "i" with stroke (ɨ), but such should not occur in Thai, Pāli, or Sanskrit.

The transliterations of วwo waen and อo ang have been included here because of their use as complete vowel symbols, but their transliteration does not depend on how they are being used and the standard simply lists them with the consonants.

Compound vowel symbols are transliterated in accordance with their constituents.

Note thatyamakkan (–๎) is represented by a spacing tilde, not a superscript tilde.

ISO 11940:1998 distinguishes the abbreviation symbolpaiyannoi (ฯ) from the sentence terminatorangkhandiao (ฯ), even though neither the national character standardTIS 620-2533 norUnicode Version 5.0 distinguishes them.Paiyannoi is transliterated asǂ andangkhandiao is transliterated asǀ. Note thatpaiyannoi,angkhandiao andangkhankhu (๚) are transliterated by the letters used forclick consonants, not by double dagger, vertical bars ordandas.

In general characters are transliterated from left to right and, where characters have the same horizontal position, from top to bottom. The vertical sequencing is in fact simply specified as tone marks andthanthakhat (–์) preceding any other marks above or below the consonant. The standard denies at the end of Section 4.2 that the combination ofsara u (◌ุ, ◌ู) andnikkhahit (◌ํ) can occur and then gives an example of it when specifying the transliteration ofnikkhahit, but does not show the transliteration of the combination. The effect of these rules is that, except fornikkhahit, all the non-vowel marks attached to a consonant in Thai are attached to the consonant in the Roman transliteration.

The standard concedes thatattempting to transpose preposed vowels and consonants may be comforting to those used to theRoman alphabet, but recommends that preposed vowels not be transposed.

For example,ภาษาไทย (RTGS: Phasa Thai) should be transliterated top̣hās̛̄āịthy andเชียงใหม่ (RTGS: Chiang Mai) toechīyngıh̄m̀.

The standard specifies the order in which the accents should be typed, but not all input systems will record accents in the order in which they are typed. Unicode specifies two normalised forms for letters with multiple accents, and transliterated text is highly likely to be stored in one of these forms. This complicates automatic back-transliteration. As Unicode-compliant processes must handle such variations correctly, the transliterations on this page have been chosen for ease of display – present day rendering systems may display equivalent forms differently.

Many fonts display novel combinations of consonants and accents badly. For example, the Institute of theEstonian Language publishes an explanation of the application of the standard toThai on the web, and with one exception this seems to be a comply with the standard. The exception is that, except for the macron, accents over consonants are actually offset to the right, giving the impression that they have been entered as the corresponding non-combining characters. The standard specifies the transliterations in code points, but someone working from this free explanation could easily deduce that the spacing forms of the tone accents should be used.

TheICU implementation, recorded in Version 1.4.1 of theCommon Locale Data Repository sponsored byUnicode,^[1] uses a prime instead of a horn in the transliteration of consonants. This affects the transliteration of ฅkho khon, ฒtho phuthao and ษso bo ruesi. ฏto patak is also transliterated differently, ast̩ rather thanṭ.

This implementation transliterates ำ as ả instead ofå to avoid ambiguity with the hypothetical Thai script sequence ะํ (sara a,nikkhahit). The ICU implementation transliterates ฺphinthu asˌinstead of to avoid problems withUnicode normalisation. This has the side effect of improving legibility when applied to an underdotted consonant.

The ICU implementation transliterates ฯpaiyannoi as‡ (double dagger) andangkhankhu as|| (two ASCII vertical bars). As the ICU implementation uses Unicode, it cannot reliably distinguishangkhandiao frompaiyannoi without a semantic analysis, and makes no such attempt.

The character sequencing of the ICU implementation is different. It transposes preposed vowels with the following consonant, and processes the marks on a consonant in the order in which they are stored in memory. (Most Thai input methods ensure that the marks are stored in bottom to top order.) It does not transpose preposed vowels with complete consonant clusters; consonant clusters cannot be identified with complete accuracy, and transposing vowels with clusters would require an additional symbol to permit reliable conversion back to the Thai script.

For example, under this implementationภาษาไทย transliterates top̣hās̄ʹāthịy andเชียงใหม่ tocheīyngh̄ım̀.

Finally, this implementation generates transliterations inUnicode Normalisation Form C (NFC).

• List of ISO transliterations
• Romanization of Lao
• Romanization of Thai
• Royal Thai General System of Transcription

• Official ISO site
• Transliteration rules on Unicode.org^{[permanent dead link]}
• Romanisation of Thai placenames in the KNAB Database of the Institute of the Estonian Language

• ISO standards
• ISO romanization
• Romanization of Thai

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