Hierarchical Relationship of Geographic Entities

Geographic entities tabulated by the U.S. Census Bureau generally are hierarchical; that 
is, in a progression from the nation to the block level. This hierarchical structure is 
derived from the legal, administrative, or areal relationships of the entities. The smallest 
level of census geography is the census block. Census blocks are defined within block 
groups, block groups are defined within census tracts, census tracts are defined within 
counties and statistically equivalent entities, counties and statistically equivalent entities 
are within states and statistically equivalent entities, states and statistically equivalent 
entities are within the nation. American Indian areas, Alaska Native areas, native 
Hawaiian areas, metropolitan areas, urban areas, and ZIP Code Tabulation Areas 
(ZCTAs(TM)) are within the nation and comprised of census blocks. Within states and 
statistically equivalent entities and comprised of census blocks are congressional 
districts, Alaska Native Regional Corporations (ANRCs), school districts, Urban 
Growth Areas (UGAs), state legislative districts (SLDs), and places. Within counties 
and statistically equivalent entities and comprised of census blocks are county 
subdivisions, voting districts (VTDs), and Traffic Analysis Zones (TAZs). In Puerto 
Rico, subbarrios are within county subdivisions and are comprised of census blocks. American 
Indian Tribal Subdivisions (AITSs) fall within American Indian/Alaska Native/Native Hawaiian
areas (AIANNHs)