Delaware County, colloquially referred to as Delco, is a county in the Commonwealth of Pennsylvania. With a population of 576,830 as of the 2020 census, it is the fifth-most populous county in Pennsylvania and the third-smallest in area. The county was created on September 26, 1789, from part of Chester County and named for the Delaware River. The county is part of the Southeast Pennsylvania region of the state.

Chester County, colloquially referred to as Chesco, is a county in the Commonwealth of Pennsylvania. It is located in the Delaware Valley region, located in the southeastern part of the state. As of the 2020 census, the population was 545,823(As of 2022), increasing by 7.1% from 498,886 in 2010. The county seat is the Borough of West Chester. The most populous of its 73 municipalities is Tredyffrin Township. The most populous boroughs are West Chester and Phoenixville. Coatesville is the only municipality in the County organized under the City form of government, a technical rather than demographic distinction. Chester County was one of the three original Pennsylvania counties created by William Penn in 1682. It was named for Chester, England. The county is part of the Southeast Pennsylvania region of the state.