This diagram shows the different geography levels for which data are available from the U.S. Census and how those levels of geography relate from the national level down to the census blocks. The geographic types connected by lines are nested within each other. For example, a line extends from counties to census tracts because a county is completely comprised of census tracts, and a single census tract cannot cross a county boundary.

While states, counties, and region are familiar geography types, census tracts, block groups, and blocks are specialized Census geographies (see box below for more information).

Excerpts from: Geography and the American Community Survey: What Data Users Need to Know

• Census tract -- are small, relatively permanent statistical subdivisions of a county or equivalent entity that are updated by local participants prior to each decennial census. Census tracts generally have a population size between 1,200 and 8,000 people, with an optimum size of 4,000 people. A census tract usually covers a contiguous area; however, the spatial size of census tracts varies widely depending on the density of settlement. Census tract boundaries are delineated with the intention of being maintained over a long time so that statistical comparisons can be made from census to census. Census tracts occasionally are split due to population growth or merged as a result of substantial population decline. Census tracts are commonly used to present information for small towns, rural areas, and neighborhoods.

• Block group -- are statistical divisions of census tracts, are generally defined to contain between 600 and 3,000 people.  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.

• Block or Census block -- are statistical areas bounded by visible features, such as streets, roads, streams, and railroad tracks, and by nonvisible boundaries, such as selected property lines and city, township, school district, and county limits and short line-of-sight extensions of streets and roads. Generally, census blocks are small in area; for example, a block in a city bounded on all sides by streets. Census blocks nest within all other tabulated census geographic entities and are the basis for all tabulated data.

For a full list of definitions -- including definitions for counties, MCDs/CCDs, places, and ZIP code tabulation areas -- please see the Census geography glossary at https://www.census.gov/programs-surveys/geography/about/glossary.html