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 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.