The Royal Thai General System of Transcription (RTGS) is the official system for rendering Thai in the Latin alphabet. It is used in road signs and government publications, and is the closest thing to a standard of transcription for Thai.
Prominent features of the Royal Thai General System include:
uses only straight letters from the latin alphabet, no diacritics
spells all vowels and diphthongs using only vowel letters: a, e, i, o, u
simple letters (a, e, i, o, u) are simple vowels (same value as in IPA)
combinations with "e" (ae, oe, ue) are simple vowels (first two same value as similar ligatures in IPA)
combinations with "a", "i", "o" (or several) are diphthongs
uses consonants as in IPA, except:
combinations with "h" are used for aspirated k, p, t (similar to letter with superscript h in IPA), to distinguish them from the separate unaspirated k, p, t
uses "ng" for IPA
uses "ch" for IPA [tʃ] and [c]
uses "y" for IPA [j]
note that transcription of consonants in final position is according to pronunciation, not spelling
Criticism
The Royal Thai General System has been criticized as inadequate for learners of Thai, particularly because of the following shortcomings:
