Most states refer to their primary political and administrative subdivisions as counties. However, there are several exceptions. For example, in Louisiana, such divisions are called parishes, and in Alaska they are called boroughs and census areas. For the purposes of this product (and the Census Bureau), parishes, boroughs, census areas, and towns (in New England) are all treated as counties. Additionally, several cities in the United States (Baltimore, Saint Louis, Carson City, and the independent cities of Virginia) are independent of counties, and therefore, are treated as county equivalents. Other exceptions include the District of Columbia, which is treated as both a state and a county.
For census purposes, the county is the primary geographic division for small area geography. Counties provide a non-overlapping set of boundaries covering the entire landscape of the nation that can effectively be subdivided into census tracts, block groups, and ultimately, blocks.