Thai is the official language of Thailand, spoken by over 20 million people (in year 2000). Standard Thai is based on the register of the educated classes of Bangkok. In addition to Central Thai, Thailand is home to other related Tai languages. Although linguists usually classify these idioms as related, but distinct languages, native speakers often identify them as regional variants of the "same" Thai language, or as "different kinds of Thai".
History Thai has undergone various historical sound changes. Some of the most significant changes, at least in terms of consonants and tones, occurred between Old Thai spoken when the language was first written and Thai of present, reflected in the orthography.
Old Thai :
Old Thai had a three-way tone distinction on live syllables (those not ending in a stop), with no possible distinction on dead syllables (those ending in a stop, i.e. either /p/, /t/, /k/ or the glottal stop which automatically closes syllables otherwise ending in a short vowel).
There was a two-way voiced vs. voiceless distinction among all fricative and sonorant consonants, and up to a four-way distinction among stops and affricates. The maximal four-way occurred in labials (/p pʰ b ʔb/) and dentals (/t tʰ d ʔd/); the three-way distinction among velars (/k kʰ ɡ/) and palatals (/tɕ tɕʰ dʐ/), with the glottalized member of each set apparently missing.
The major change between old and modern Thai was due to voicing distinction losses and the concomitant tone split. This may have happened between about 1300 and 1600 AD, possibly occurring at different times in different parts of the Thai-speaking area. All voiced–voiceless pairs of consonants lost the voicing distinction:
Source : http://en.wikipedia.org/wiki/Thai_language