The region is comprised of 10 counties: Berks, Bucks, Chester, Delaware, Lancaster, Lebanon, Lehigh, Montgomery, Northampton, and Philadelphia.
The three original counties, created by William Penn in 1682, include Philadelphia, Bucks, and Chester. Some 50 years later the next county - Lancaster - was carved out of Chester County in 1728. Berks County was formed from Chester, Lancaster, and Philadelphia counties in 1752 and the same year Northampton County was created from Bucks County.
In 1784 Montgomery County was carved from Philadelphia, and in 1789 Delaware County was formed from Chester.Lehigh County was created from part of Northampton County in 1812, followed by Lebanon County in 1813, which was created from Dauphin and Lancaster counties.
When searching for records during these time periods it is good to keep in mind these changing county formation dates. The farms didn't move; the boundaries around them did!