text: History There have been four versions of the RTGS, those promulgated in 1932, 1939, 1968 and 1999. The general system was issued by the Ministry of Public Instruction in 1932, and subsequent issues have been issued by the Royal Institute of Thailand.
  Table of changes
  1932 version
  The general system was set up by a committee of the Ministry of Public Instruction on the following principles:
  The general system should be expandable to the precise system.
  The general system should be based on pronunciation, and one sound should be represented by one symbol or letter.
  The general system should be in consonance with the principles of Thai grammar, orthography, and pronunciation.
  In selecting symbols or letters, account should be taken of existing types for printing and typewriting and of existing systems of transcription.
  The committee considered that for the general system, tone and quantity marks were unneeded. They would be provided for the precise system. The marks are accents above the vowels, one reason that the vowel symbols used to have no marks above them.
  1939 version
  The 1939 issue allowed short vowels to be marked with a breve (˘) where expedient. By contrast, the ALA-LC transliteration uses the 1939 version with the addition of a macron (¯) for long vowels and a spiritus asper (ʽ) to transliterate อ  as a consonant.
  The changes in vowel notation copied existing usage (æ, œ)
  and IPA notation (æ, ǫ).