Counties and their equivalents are the primary political and administrative divisions of States. These areas are called parishes in Louisiana. In Alaska, 27 boroughs and "census areas" are treated as county equivalents for census purposes. Several cities (Baltimore, MD, St. Louis, MO, Carson City, NV, and 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. That part of Yellowstone National Park in Montana is treated as a county equivalent. The District of Columbia has no primary divisions, and the entire area is considered equivalent to a county for statistical purposes. Kalawao County, HI, is combined with Maui County for statistical purposes.

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.