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).
For a full list of definitions -- including definitions for counties, MCDs/CCDs, places, and ZIP code tabulation areas -- please see the Census geography glossary here.