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 Several geographic relationships apply throughout the 21 CCD States. The most common pattern is a CCD composed of one or more whole census tracts/BNAs; however, there are many instances where a census tract/BNA consists of two or more CCDs, or more rarely, of one CCD and part of another. In a very few cases, there is no geographic relationship between the two sets of areas.

The MCDs of the counties in the nine northeastern States are both stable geographic entities and well-known units of local government. As a result, they often figure as the geographic basis for census tracts/BNAs. An MCD generally consists of one or more census tracts/BNAs, and the boundaries of census tracts/BNAs usually do not cross the boundaries of any MCD or MCD equivalent.

By contrast, in the midwestern and southern States, the MCD boundaries usually do not coincide with groups of census tracts/BNAs, except where both sets of boundaries follow a physical feature. However, there are some instances where census tract boundaries follow nonvisible MCD lines, because the census tract criteria at one time permitted this situation.

The 1990 criteria for delineating block groups within census tracts and BNAs allowed block group boundaries to follow nonvisible MCD boundaries only in the northeastern States. When a CCD boundary was not a census tract/BNA boundary, it was preferred as a block group boundary. Where an MCD boundary, or occasionally a CCD boundary, split a physical block, the Census Bureau assigned an alphabetic suffix to identify separately each tabulation block created by the location of the county subdivision boundary.

The Federal Office of Management and Budget (OMB) establishes the standards for, and then defines, metropolitan areas (MAs) either as County Subdivisions8-37