Block groups are statistical divisions of census tracts used by the United States Census Bureau to present data and control block numbering. They are the smallest geographic unit for which the bureau publishes sample data.
A block group typically covers a contiguous area. However, it is possible for block groups to consist of blocks that do not share a common boarder. The size of a block group can vary, yet generally they are defined to contain between 600-3,000 people. There is at least one block group in each census tract, and those within a census tract are given a unique code. However, block groups never cross state, county, or census tract boundaries.
Tribal Block Groups: Tribal census tracts and tribal block groups are defined by federally recognized American Indian reservations or off-reservation trust land. These types of geographic divisions can cross state and county boundaries and may be completely different than the census tracts and block groups defined by the state and county. A single tribal block group is defined by a population less than 1,200. Populations over 1,200 result in multiple tribal block groups.
Block Group Codes: Each block group within a census tract has a unique numeric code using the digits 0-9. This is used to differentiate between block groups, as well as provide insight to certain characteristics of the block group. A code beginning with a zero represents a block group that only contains water area. To distinguish a county-based block group from a tribal block group, tribal block groups use an alphabetic code of the characters A-K. (This excludes "I" which could be confused with the number "1".)