All census tracts are subdivided into smaller geographic regions called block groups. The block group is, as the name suggests, a cluster of census blocks (the smallest area for which the Census Bureau provides data) that combine to form a census tract. Block groups never cross boundaries of states, counties, or statistically equivalent entities, except for Block Groups delineated by American Indian tribal authorities. Block groups never cross the boundaries of census tracts, but may cross the boundary of any other geographic entity required as a census block boundary. Block groups generally contain between 600 and 3,000 people, with an optimum size of 1,500 people. Block groups on American Indian reservations, off-reservation trust lands, and special places must contain a minimum of 300 people.
Block groups have 1-digit codes (0-9) that reflect the first digit of the 3-digit identifiers of the blocks they contain. For example, the figure contains a block group including the blocks #501-512.