This document specifies a set of Label Generation Rules (LGR) for the Thai script for the second level domain or domains identified above. The starting point for the development of this LGR can be found in the related Root Zone LGR [RZ-LGR-Thai]. The format of this file follows [RFC 7940]. This LGR is adapted from the “Reference LGR for the Second Level for the Thai Script” [Ref-LGR-und-Thai], for details, see Change History below.
For details and additional background on the Thai script, see “Proposal for a Thai Script Root Zone LGR” [Proposal-Thai].
The repertoire contains 68 code points for letters used in the Thai language. In addition, three sequences have been defined. The sequence U+0E4D U+0E32 was defined to replace the disallowed U+0E33 SARA AM and to facilitate implementation of the WLE rule follows-consonant-tone as a context rule. The other two sequences were defined to restrict U+0E45 LAKKHANGYAO from appearing in any context other than these sequences. Accordingly, while U+0E45 is not listed by itself this brings the total of code points avaliable for letters to 69.
The repertoire is a subset of [Unicode 11.0.0]. For details, see Section 5, “Repertoire” in [Proposal-Thai]. (The proposal cited has been adopted for the Thai script portion of the Root Zone LGR.)
For the second level, the repertoire has been augmented with the ASCII digits, U+0030 to U+0039; Thai digits, U+0E50 to U+0E59; U+002D HYPHEN-MINUS; the abbreviation mark U+0E2F THAI CHARACTER PAIYANNOI; and the repetition mark U+0E46 THAI CHARACTER MAIYAMOK. Also, a special sequence using the abbreviation mark has been added for a total of 95 repertoire elements.
Repertoire Listing: Each code point or range is tagged with the script or scripts with which the code point is used and one or more other character categories. For each repertoire element, one or more references document sufficient justification for inclusion in the repertoire; see the “References” below. For code points that are part of the repertoire, comments identify the language using the code point, as well as alternate names of some code points.
The variants defined in this LGR are limited to those required for use in zones not shared with any other script. However, because it does not share cross-script variants with other script LGRs, this LGR can be mixed easily with other LGRs in the same zone.
This LGR defines no variants for letters, see Section 6, “Variants” in [Proposal-Thai].
Digit Variants: All Thai digits are treated as semantic variants of the corresponding common (ASCII) digits. By transitivity, they are also semantic variants of any native digits in scripts that also include the common digits. Such variant relations are deemed to exist implicitly by transitivity but are not listed explicitly in each reference LGR. Instead, if needed, they are applied by using the Common LGR in label processing.
In particular, Thai digit ZERO is a cross-script homoglyph or near homoglyph of digit ZERO in many other scripts; these are already implicit semantic variants by transitivity and therefore not listed here.
There is a strong resemblance between Thai and Khmer digits, as well as with certain Lao digits.
To keep digit variant sets manageable in zones where multiple scripts are present, no attempt has been made at identifying cross-script variants among digits of different numeric value or between a digit in one script and a letter in another, such as between digit zero and Latin letter ‘o’. Other mechanisms may be required to prevent homograph labels.
The Thai Script is an abugida in which consonant–vowel sequences are written as a unit: each unit is based on a consonant letter, and vowel, tone mark or diacritic notation are secondary. It is written with the combining marks stacked above or below the base consonant, like diacritics in European languages. However, although the concepts are quite similar, the implementations are significantly different.
Consonants: There are 44 characters that are classified as consonants; code points from this subset have been given the tag “cons”. See Section 5.1, “Consonants” in [Proposal-Thai].
Vowels: The 18 vowel symbols pronounced after a consonant are non-sequential: they can be located before (lv) , after (fv), above (av) or below (bv) the consonant, or in a combination of these positions, code points from this subset have been given the tags “fv1”,"fv2", “fv3”, “av”, “bv”, or “lv”. There are three code point sequences defined that include vowels. (Code point sequences do not carry tag values; instead, for code point sequences the subset values are identified in comments). See Section 5.2, “Vowels” in [Proposal-Thai].
Tones: There are 5 phonemic tones: mid, low, falling, high, and rising. These 5 tones are represented by 4 tone marks plus the absence of a mark. Code points from this subset have been given the tag “tone”. See section 5.3, “Tone Marks” in [Proposal-Thai].
Diacritical Marks: There are 3 above diacritic symbols that have been included here and given the tag “ad”. They differ in their frequency and purpose of usage. See also the discussion in Section 5.,4 “Diacritics” in [Proposal-Thai].
A fourth above diacritic, U+0E4E YAMAKKAN, has been excluded from the Root Zone LGR repertoire because it is rarely used in Modern Thai or even in older Pali manuscripts; it is more common to replace it with U+0E3A PHINTHU, a below diacritic, which has been given the tag “bd”. Moreover, excluding U+0E4E YAMAKKAN also eliminates the chance of confusion between U+0E4E YAMAKKAN and U+0E4C THANTHAKHAT. Both look similar, are always placed at the same position in the word cell, and they are normally displayed in a small size.
Abbreviation Mark: The abbreviation mark (or ellipsis) U+0E2F PAIYANNOI should not be at the beginning position and must end the label. An exception to this rule has been provided for by a special sequence.
Repetition Mark: The repetition mark U+0E46 MAIYAMOK repeats anything before it. Therefore, it should not be at the beginning position. The repetition mark must not be followed by any combining mark, or follow a leading vowel to prevent unstable rendering. A repetition mark may be repeated. A context rule has been defined to limit the repetition mark to acceptable context, however, some of the restrictions are taken care of by other rules, for example, the rule that leading vowel must be followed by a consonant.
Thai Digits: U+0E50 to U+0E59 are a set of Thai-specific digits. They are used in alternation with the European (common) digits.
Common Digits: U+0030 to U+0039 are the set of digits from the ASCII range.
The LGR includes the rules and actions to implement the following restrictions, some of which are mandated by the IDNA protocol. They are marked with ⍟ and included here by default.
This LGR includes the complete set default actions for LGRs as well as the action needed to invalidate labels with misplaced combining marks. They are marked with ⍟. For a description see [RFC 7940] and [150]. An additional action is triggered by mixed digit labels.
The rules provided in this LGR as described in Section 7 of [Proposal-Thai] reasonably restrict labels so that they conform to Thai syllable structure. These constraints are exclusively presented as context rules.
The rules are:
The LGR in this document has been adapted from the corresponding Reference LGR for the Second Level. The Second Level Reference LGR for the Thai Script was developed by Michel Suignard and Asmus Freytag, based on the Root Zone LGR for the Thai script and information contained or referenced therein; see [RZ-LGR-Thai]. Suitable extensions for the second level have been applied according to the [Guidelines] and with community input. As part of that process, features have been removed or modified as appropriate for the target language. The original proposal for a Root Zone LGR for the Thai script, that this LGR is based on, was developed by the Thai Generation Panel. For more information on methodology and contributors to the underlying Root Zone LGR, see Sections 4 and 8 in [Proposal-Thai], as well as [RZ-LGR-Overview].
This version modifies the Second Level Reference LGR for the Thai Script [Ref-LGR-und-Thai] by injecting the complete (maximal) set of cross-repertoire variants, including any imposed by transitivity. Injected variants are marked by special comments. Any character classes, context and WLE rules required for testing are also imported. In all other respects, description or annotations have not been updated to reflect the injected variants. Any LGR elements that might pose a possible conflict when the LGR is used with the Common LGR for cross-repertoire variant resolution are clearly marked. All modifications are carried out mechanically; there may be an occasional context rule or similar that is injected because its definition relates to some repertoire element even though it can never be triggered by any label valid under the modified LGR.
The following general references are cited in this document:
For references consulted particularly in designing the repertoire for the Thai Script for the second level please see details in the Table of References below.
Reference [0] refers to the Unicode Standard version in which corresponding code points were initially encoded. References [100] and [101] correspond to sources given in [Proposal-Thai] for justifying the inclusion of for the corresponding code points. Entries in the table may have multiple source reference values. In the listing of whole label evaluation and context rules, reference [150] indicates the source for common rules.
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