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 (legal entities) and census areas (statistical entities), both of which serve as the geographic equivalents of counties in Alaska. Although CSAs are similar to CCDs, there are enough differences to warrant treating them as a separate type of county subdivision.

The decennial censuses of population and housing identify and present data for all types of county subdivisions in every State (see ). Certain types of MCDs figure in the population estimates programs, the Census of Governments, and, on a selective basis, the economic censuses. This chapter discusses the county subdivisions of the United States; for information on similar geographic entities in Puerto Rico and the Outlying Areas, see.

The decennial population censuses always have sought to identify county subdivisions as a primary geographic entity. The first census (1790) reported population counts for MCDs (towns, townships, and other units of local government), and this practice continued throughout subsequent censuses. For States in which local governmental units did not exist below the county level, various administrative units or other area designations were used, such as companies, districts, hundreds, remainder of county, and eastern (or western) portion of county. Although this chapter does not trace the history of such alternate and short-lived entities in detail, the final section of this chapter provides some background information about the different kinds of MCDs used for the 1990 decennial census.

Starting with the 1950 census, there have been four significant developments in the Census Bureau’s treatment of county subdivisions: (1) the replacement of MCDs with CCDs in 21 States, (2) the establishment of the UT as a standard geographic entity in 9 States, (3) the subdivision of Alaska into CSAs, and (4) the establishment of the Boundary and Annexation Survey (BAS) program. 8-2County Subdivisions