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Allegheny County, PA was formed on September 24, 1788 out of Westmoreland and Washington counties.  It was named for the Allegheny River, which joins with the Monongahela at Pittsburgh to form the Ohio River.  Pittsburgh, the county seat, was laid out 1764 and incorporated as a city in 1816.  An important center of the colonial frontier, Pittsburgh became an important industrial and manufacturing city, leading the United States in the production of steel, glass, and other materials beginning in the late-19th and continuing for more than a century.