Central Thailand is a region of Thailand which consists of 21 provinces, covering the broad alluvial plain of the Chao Phraya River. It is separated from Northeastern Thailand (Isan) by the Phetchabun mountain range, and another mountain range separates it from Myanmar to the west. In the north it gently changes into the more hilly 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 (Krungthep Mahanakhon). Central Thailand is the most populated region in the country.