Pittsburgh 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 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. This vital link of the Atlantic coast and Midwest through the mineral-ri ... more on Wikipedia