1940 - Block data were published for selected housing statistics in 191 cities with 50,000 or more inhabitants. (GARM 11-5)

1990 - 1990 was the first census in which the entire U.S. was blocked.

1870-1880 - Districts of enumeration (later called enumeration districts) began. (GARM 11-2)

Enumeration districts "continued to be the smallest geographic unit for which census data were available until census blocks were introduced for larger places in the 1940 census; even then, because blocks were numbered only in limited areas, EDs continued to be used as a collection and reporting unit in decennial censuses through the 1980 census". (GARM 11-3)

1970 - "In the block numbered areas, the Census Bureau devised the block group (BG) as a data tabulation and publication unit equivalent to the ED in non-block numbered areas. Originally referred to as quarter tracts, BGs were subdivisions of a census tract or block numbering area." (GARM 11-6)

1990 - Block groups have "largely replaced the earlier enumeration district (ED) as a small-area geographic unit for purposes of data presentation". (GARM 11-1)

1990 - 1990 was the first census in which the entire U.S. was block grouped.

1910-1930 - Census tract or district data were collected for some large cities. (GARM 10-1)

1940 - Census tracts were officially adopted. (GARM 10-3)

1940 - Block areas (later block numbering areas) were used in areas without census tracts. (GARM 10-3 - 10-4)

1960 - Block areas were renamed block numbering areas. (GARM 10-4)

1990 - 1990 was the first census in which the entire U.S. was tracted.

2000 - Block numbering areas were renamed census tracts.

Examples of county subdivisions include: minor civil division (MCD) (township, town); census county division (CCD); unorganized territory (UT); census subarea (CSA)(Alaska); election precinct, magisterial district, parish governing authority district, supervisors' district, election district, plantation, assessment district, etc. (GARM 8)