text: Relationship to precise system The precise system was issued along with the general system in 1939. A transliteration in the precise system could be converted to the general system by doing the following:
  Removing parenthesised character
  Replacing ʽ and hʽ by h
  Removing length and tone markings
  Removing ḥ, which corresponds to ะ , which may be viewed as a length mark
  Removing the character distinguishing dots below and primes
  Changing ay and aiy to ai except before vowels
  Changing č to čh
  Changing ie to ia, uo to ua and ưœ to ưa
  The last set of changes removes a graphic distinction between vowels in closed syllables and vowels in open syllables.
  The h is added to č in the general system to make it easier to read. When the diacritic was subsequently removed, the h was justified as avoiding the misreading of the transliteration as  or  rather than the correct .
  1968 version
  The 1968 version removed diacritics, including the horn of ư and replaced the ligatures æ and œ by ae and oe. While that is more suitable as the standard transliteration for maps, it removed the contrast between the transcriptions of จ  and ช , อึ  and อุ , เอือ  and อัว , and โอ  and ออ .