2020 Census block groups are clusters of blocks within the same census tract that have the same first digit of their 4-digit census block number; that first digit is the block group number. Block group numbers have a valid range of zero (0) through nine (9). Block groups beginning with a zero generally are in coastal and Great Lakes water and territorial seas. A block group usually covers a contiguous area. Each census tract contains one or more block groups and block groups have unique numbers within a census tract. Within the standard census geographic hierarchy, block groups never cross county or census tract boundaries, but may cross the boundaries of county subdivisions, places, urban areas, voting districts, congressional districts, and AIANNH areas. Block groups delineated for the 2020 Census generally contain 600 to 3,000 people.