Islamabad is the capital of Pakistan and the tenth largest city in the country. The population of the city has increased from 100,000 in 1951 to 1.21 million in 2009. The Rawalpindi/Islamabad Metropolitan Area is the third largest in Pakistan with a population of over 4.5 million inhabitants. Islamabad is located in the Pothohar Plateau in the north of the country, within the Islamabad Capital Territory. The region has historically been a part of the crossroads of Punjab and Khyber Pakhtunkhwa with Margalla pass acting as the gateway between the two regions. The city was built during the 1960s to replace Karachi as Pakistan's capital.