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 available for very small units of geography (the census block) as well as at the next higher levels (the BG and the census tract or BNA). The availability of these low-level geographic entities provides extensive flexibility for data users to obtain counts for geographic units of specific interest to them. These increased capabilities should stimulate much interest in decennial census data throughout the 1990s and beyond. As a result, many of the principles, concepts, and criteria described in this chapter, and in greater detail in the subsequent ones, will find renewed application, while others may become obsolete. Almost certainly, new concepts will emerge. The basic concepts embodied in this chapter, however, will serve to guide the continuing evolution of Census Bureau geography. 2-34Geographic Overview