Allegheny County was officially created on 24 September 1788 from parts of Washington, Westmoreland and Northumberland counties. It was formed due to pressure from settlers living in the area around Pittsburgh, which became the county seat in 1791. The county originally extended all the way to the shores of Lake Erie and became the "mother county" for most of what is now northwestern Pennsylvania. By 1800, the county's current borders were set.