Austin is the capital of the U.S. state of Texas and the county seat and most populous city of Travis County, with portions extending into Hays and Williamson counties. Incorporated on December 27, 1839, it is the 11th-most populous city in the United States, the fourth-most populous city in the state after Dallas, San Antonio and Houston, and the second-most populous state capital city after Phoenix, the capital of Arizona. It has been one of the fastest growing large cities in the United States since 2010. Downtown Austin and Downtown San Antonio are approximately 80 miles (129 km) apart, and both fall along the Interstate 35 corridor. This combined metropolitan region of San Antonio–Austin has approximately 5 million people. Austin is the southernmost state capital in the contiguous United States and is considered a Gamma-level global city as categorized by the Globalization and World Cities Research Network.