This file contains Label Generation Rules (LGR) for the Thai script as would be appropriate for the Root zone. For more details on this LGR see "Proposal for a Thai Script Root Zone LGR [Proposal]"
In addition to the 68 code points according to Section 5 “Repertoire” in [Proposal], three sequences have been defined. The sequence U+0E4D U+0E32 was defined to replace the disallowed U+0E33 (THAI CHARACTER SARA AM) and to facilitate implementation of WLE rule follows-consonant-tone as a context rule. The other two sequences were defined to restrict U+0E45 (THAI CHARACTER LAKKHANGYAO) from appearing in any context other than these sequences. Accordingly, while U+0E45 is not listed by itself it brings the total of distinct code points to 69.
According to Section 6 "Variants", in "[Proposal]", this LGR defines no variants.
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.
There are 44 characters that are classified as consonants, code points from this subset have been given the tag "cons".
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 tag "fv1","fv2","fv3","av","bv","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 indentified in comments).
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"
There are 3 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 in [Proposal].
The Thai GP decided to exclude a fourth above diacritic, U+0E4E (YAMAKKAN), from the 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). 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.
The LGR includes the set of required default WLE rules and actions applicable to the Root Zone and defined in [MSR-2].
The rules provided in this LGR as described in Section 7 of [Proposal] reasonably restrict labels so that they conform to Thai syllable structure. These constraints are exclusively presented as context rules.
The rules are:
For methodology and contributors, see Sections 4 and 8 of [Proposal].
Reference [0] refers to the Unicode Standard version in which corresponding code points were initially encoded. Reference [100] corresponds to a source given in [Proposal] for justifying the inclusion of for the corresponding code points. Single code point or ranges may have multiple source reference values.
In addition the following references are cited in this document:
For more details for references [100] and up and [0] and up refer to the Table of References below.
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