There are 18 congressional districts in Pennsylvania, and these districts are referred to by their number. Congressional districts may follow county lines, but it is common for a congressional district to cover only a portion of one or more counties. For example, as of 2016, the 17th Congressional District includes Schuylkill County and parts of Monroe, Carbon, Northampton, Lackawanna and Luzerne Counties.
In Pennsylvania, four congressional districts represent Philadelphia. These districts are the 1st, 2nd, 8th and 13th. Another congressional district is the 12th, which covers Beaver County and parts of Cambria, Somerset, Westmoreland, Allegheny and Lawrence counties.
The U.S. Census determines the number of congressional districts each state receives. After the 1990 Census, Pennsylvania had 21 congressional districts. However, the state's population has declined and it lost seats following both the 2000 and 2010 censuses.
If there is a change in the number of congressional seats, a state must undergo a re-districting process. During this time, new boundary lines for districts are drawn. The state legislature oversees this process, and as a result, the political party that controls the state legislature has the ability to shape the new district boundaries. Political parties can create districts that ensure the majority party retains its advantage in future elections.