 Eight of the ten most populous counties are in the southeastern portion of the state, including four out of the top five, and eight of the top ten most populous counties are in either the Philadelphia or Pittsburgh Metropolitan Statistical Areas.The Federal Information Processing Standard (FIPS) code, used by the United States government to uniquely identify counties, is provided with each entry.FIPS codes are five-digit numbers; for Pennsylvania the codes start with 42 and are completed with the three-digit county code.