There are many systems in use for the Romanization of Thai. Some place more emphasis on the etymology of a word, others on the pronunciation. This table compares three standards; ISO 11940, the Royal Thai General System of Transcription (RTGS), and the American Library Association - Library of Congress (ALA-LC) standard. ISO 11940 and the ALA-LC standard are transliteration systems, the RTGS is technically a transcription system (see Transliteration vs. Transcription for a discussion of the differences). In practice, these standards are applied inconsistently, or a combination of them will be used in a single document or on a single sign.

The RTGS and the ALA-LC transcribe consonants differently depending on whether they are at the beginning or the end of a syllable. In the table, both Latin equivalents are given, separated by a slash / .

Consonants

ThaiISO 11940RTGSALA-LC
  k k / k k / k
  k̄h kh / k kh / k
  ḳ̄h kh / k kh / k
  kh kh / k kh / k
  k̛h kh / k kh / k
  ḳh kh / k kh / k
  ng ng / ng ng / ng
  c ch / t čh / t
  c̄h ch / - ch / t
  ch ch / t ch / t
  s s / t s / t
  c̣h ch / - ch / t
  y / n y / n
  d / t d / t
  t / t t / t
  ṭ̄h th / t th / t
  ṯh th / t th, d / t
  t̛h th / t th / t
  n / n n / n
  d d / t d / t
  t t / t t / t
  t̄h th / t th / t
  th th / t th / t
  ṭh th / t  
  n n / n n / n
  b b / b b / p
  p p / p p / p
  p̄h ph / - ph / p
  f / - f / p
  pj ph / p ph / p
  f f \ p f / p
  p̣h ph / p ph / p
  m m / m m / m
  y y / - y / -
  r r / n r / n*
  v rue, ri, roe / - rư, ri, rœ / -
  l l / n l / n
  ł lue lu
  w w w / -
  ṣ̄ s / t h / -
  s̛̄ s / t s / t
  s / t s / t
  h / -
  l / n l / n
  x o ’ (ayn) / -**
  h / - h / -

Vowels and Punctuation

ThaiISO 11940RTGSALA-LC
  a a a
  a a
  ā a ā
  å am am
  i i i
  ī i ī
  ue ư
  ụ̄ ue ư̄
  u u u
  ū u ū
  e † (see below) ̄e
  æ † (see below) ǣ
  o † (see below) ō
  ı ai ai
  † (see below) ai
  rư̄, lư̄
  ǂ    
  «
  ̆    
  ̀    
  ̂    
  ́    

† The RTGS and ALA-LC transcribe some vowel symbols differently depending on the context in which they appear, as follows. (In this chart, the hyphen symbol - represents a consonant.)

Vowels (contextual)

ThaiRTGSALA-LC
  - a *
  an *
  - e e
  - e e
  - e ̄e
  - ae æ
  - ae ǣ
  - o o
  - o ō
  - o ǫ
  - o ’ (ayn) / -**
  - oe œ̄
  - oe œ
  - oe œ
  - ia ia
  - ia īa
  - uea ưa
  - uea īa
  - ua ua
  - ua ūa
  -- ua ūa
  - ai ai
  - ai ai
  - ai āi
  - ai ai
  - ai āi
  - ao ao
  - ao āo
  - ui ui
  - oi ōi
  - oi ǭi
  - oei œ̄i
  - ueai ư̄ai
  - uai ūai
  - io iu
  - eo eo
  - eo eo
  - aeo
  - aeo ǣo
  - iao īeo

* When ร follows another consonant and ends a syllable, it is romanized n, and the inherent vowel of the preceding consonant is represented by ǭ (e.g., นคร nakhǭn). When รร follows another consonant and no other pronounced consonant follows in the same syllable, it is romanized an (e.g., สวรรค sawan), but if a pronounced consonant follows, it is represented by a (e.g., กรรม kam).
**In four common words อ occurs preceding another consonant to mark a certain tone and is then not romanized.