group number as the first digit of the block number. Thus census blocks are subdivisions of, and nest within, a specific block group.
The relationship of census tract/BNAs to county subdivisions (census county divisions and minor civil divisions) and places (incorporated places and census designated places) varies. Many States have incorporated places such as cities, boroughs, and villages, and minor civil divisions (MCDs) such as towns and townships. The boundaries of some of these governmental units are not well known locally or shift frequently as a result of annexations. In these States, the Census Bureau discourages the use of these governmental unit boundaries as census tract/BNA boundaries; data users will find that the layout of the governmental units seldom corresponds to the census tract/BNA framework. In the New England States, where governmental unit boundaries change infrequently and are well known locally, data users generally will find a nesting relationship between census tracts/BNAs and governmental units. Wherever possible, the Census Bureau has continued the practice of encouraging congruency between census county divisions (CCDs) and census tracts/BNAs, and does so by revising the CCD boundaries when a census tract/BNA needs to change.
The areas and boundaries of other census geographic entities bear no geographic relationship to census tracts/BNAs because there are different reasons for their establishment. Their boundaries, therefore, may or may not conform to those of the census tracts/BNAs. Such entities include census designated places (CDPs), voting districts, school districts, American Indian reservation and subreservation areas, Alaska Native villages, and congressional districts. Many data users inquire about the geographic relationship between census tracts/BNAs and ZIP Code areas (geographic entities that approximate the assignment of ZIP Codes by the U.S. Postal Service)—census tracts/BNAs rarely correspond to ZIP Code areas.
