Counties and their equivalents, 3,323 in all, are the primary political and administrative divisions of States. These areas are called parishes in Louisiana. In Alaska, boroughs and “census areas” are treated as county equivalents for census purposes. Several cities (Baltimore, MD, St. Louis, MO, Carson City, NV, and over 40 cities in Virginia) are independent of any county organization and, because they constitute primary divisions of their States, are accorded the same treatment as counties in census tabulations. Finally, since the District of Columbia has no primary divisions, the entire area is considered equivalent to a county for statistical purposes.

The Economic Census of Island Areas also has geographies treated as county equivalents for census purposes. These include Islands and Districts in American Samoa, Districts in Guam, Municipalities in the Northern Mariana Islands, Municipios in Puerto Rico, and Islands in the Virgin Islands.

Counties are identified by a 3-digit Federal Information Processing Standards (FIPS) code, which is sequenced alphabetically within state, except for the independent cities, which follow the listing of counties within state.