Central Thailand (central plain) is a region of Thailand, covering the broad alluvial plain of the Chao Phraya River. It is separated from northeast Thailand (Isan) by the Phetchabun mountain range. The Tenasserim Hills separate it from Myanmar to the west. In the north it is bounded by the Phi Pan Nam Range, one of the hilly systems of northern Thailand. The area was the heartland of the Ayutthaya Kingdom, and is still the dominant area of Thailand. Central Thailand contains the Thai capital of Bangkok and it is the most populated region in the country.
The grouping of Thai provinces into regions follow two major systems, in which Thailand is divided into either four or six regions. In the six-region system, commonly used in geographical studies, central Thailand extends from Sukhothai and Phitsanulok Provinces in the north to the provinces bordering the Gulf of Thailand in the south, excluding the mountainous provinces bordering Myanmar to the west and the coastal provinces of the east. The four-region system includes provinces only as far north as Chainat, Sing Buri, and Lopburi, and extends west and east to the borders of Myanmar and Cambodia.
The central region is divided into 22 provinces, which includes the following:
The eastern region is sometimes listed as a separate region distinct from central Thailand – sometimes only the four coastal provinces, sometimes the above list excluding Nakhon Nayok. None of these regions are actually administrative subdivision, they are only geographical or statistical groupings.
In Thailand Central Thai are the ethnicity that is the dominant ethnic group and language standard for forming standard Thai, rather than the other ethnic peoples in Thailand and their languages or dialects. Thai language is thus Central Thai language to correctly name it.