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 The Census Bureau discourages the use of governmental unit boundaries as census tract/BNA boundaries because of the need to freeze the census tract/BNA boundaries at the time of census block numbering, which occurs several years before a decennial census. Once the census tract/BNA boundaries are frozen, any changes to the governmental unit boundaries, whether as a result of annexations, detachments, or mapping corrections, result in the census tract/BNA boundaries continuing to follow the former (incorrect) location of the governmental unit boundary. The result can be the loss of the intended nesting relationship between census tracts/BNAs and the governmental unit.

Holding the boundaries of counties (or statistically equivalent entities) as census tract/BNA boundaries is a fundamental requirement of the census tract/BNA programs. Census tracts and BNAs are subdivisions of counties, and they nest within counties. Because the Census Bureau needed to have the census tracts/BNAs delineated before numbering the 1990 census blocks, it had to approve the census tract/BNA plans several years beforehand. Knowing that it would be necessary to update some county or State boundaries after the establishment of census tracts/BNAs and the assignment of 1990 census block numbers (but before data tabulation), the Census Bureau designed a method to accommodate the latest (January 1, 1990) State and county boundary changes. Changes in the 1990 census tract/BNA boundaries ordinarily would require renumbering some census blocks, yet the Census Bureau had to design a method of updating State or county boundaries without changing any census block numbers. As a result, the Census Bureau recognized as census tract/BNA boundaries both the superseded and the corrected State or county boundaries. The result was the formation of (usually) small census tracts/BNAs, often containing little or no population or housing units, that represented the territory affected by the State or county boundary update (see ). Census Tracts and Block Numbering Areas10-13