Block Groups (BGs) are statistical divisions of census tracts, are generally defined to contain between 600 and 3,000 people, and are used to present data and control block numbering. A block group consists of clusters of blocks within the same census tract that have the same first digit of their four-digit census block number. For example, blocks 3001, 3002, 3003, . . ., 3999 in census tract 1210.02 belong to BG 3 in that census tract. Most BGs were delineated by local participants in the Census Bureau's Participant Statistical Areas Program. The Census Bureau delineated BGs only where a local or tribal government declined to participate, and a regional organization or State Data Center was not available to participate.
A BG usually covers a contiguous area. Each census tract contains at least one BG, and BGs are uniquely numbered within the census tract. Within the standard census geographic hierarchy, BGs never cross state, county, or census tract boundaries but may cross the boundaries of any other geographic entity. Tribal census tracts and tribal BGs are separate and unique geographic areas defined within federally recognized American Indian reservations and can cross state and county boundaries (see "Tribal Census Tract" and "Tribal Block Group"). The tribal census tracts and tribal block groups may be completely different from the census tracts and block groups defined by state and county.
Block Group Codes—BGs have a valid code range of 0 through 9. BGs beginning with a zero only contain water area and are generally in coastal and Great Lakes water and territorial seas, but also in larger inland water bodies. For the 2010 Census, a block group 0 for the water portion can be delineated in any census tract and not just those census tracts also defined to only include water area. This is a change from Census 2000, when block groups coded 0 only existed in census tracts with a code of 0. To differentiate between county-based block groups and tribal block groups, the codes for tribal block groups use an alphabetic character (see "Tribal Block Group").