Pittsburgh is the second-largest city in the U.S. Commonwealth of Pennsylvania and the county seat of Allegheny County. Regionally, it anchors the largest urban area of Appalachia and the Ohio River Valley, and nationally, it is the 22nd-largest urban area in the United States. The estimated population of the city in 2009 was 311,647, while the seven-county metropolitan area was estimated at 2,354,957. Downtown Pittsburgh retains substantial economic influence, ranking at 25th in the nation for jobs within the urban core and 6th in job density. The characteristic shape of Pittsburgh's central business district is a triangular tract carved by the confluence of the Allegheny and Monongahela rivers, which form the mighty Ohio River. The city features 151 high-rise buildings, 446 bridges, two inclined railways, and a pre-revolutionary fortification. Pittsburgh is known colloquially as "The City of Bridges" and "The Steel City" for its many bridges and former steel manufacturing base.