Pittsburgh (/ˈpɪtsbərɡ/ PITS-burg) is the second-largest city in the U.S. state of Pennsylvania with a population of 305,842 and the county seat of Allegheny County. The Combined Statistical Area (CSA) population of 2,659,937 is the largest in both the Ohio Valley and Appalachia and the 20th-largest in the U.S. Pittsburgh is known as both "the Steel City" for its more than 300 steel-related businesses, as well as "the City of Bridges" for its 446 bridges. The city features 30 skyscrapers, 2 inclines, a pre-revolutionary fortification and the source of the Ohio River at the confluence of the Monongahela and Allegheny Rivers.