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 Throughout most of the 1800s, the smallest data unit for which the Census Bureau collected information was the area assigned to an individual enumerator, and the smallest area for which it reported data corresponded to geographic entities such as wards, communities, and townships. Histories of the early U.S. censuses contain very little precise information about how these entities were subdivided for enumeration. Many of the assignments were verbal descriptions based on legally defined entities, community names, and major physical features. Some enumerators developed their own geographic solutions by drafting sketch maps and by writing descriptions of their enumeration area boundaries.

Over time, the instructions for enumerators became more specific; they revealed an increasing concern for the boundaries, size, and identification of geographic subdivisions. For the 1870 census, the Census Office (predecessor of the Census Bureau) lowered the maximum size of the subdistricts used for enumeration from 50,000 to 6,000 people. For the 1880 census, this number was reduced to 4,000, resulting in approximately 28,000 districts of enumeration (later called enumeration districts, or EDs). This census also was the first in which the Census Office provided maps of EDs for use in taking the census.

For 1910, the Census Bureau began to delineate EDs to follow the boundaries of legally or administratively defined entities such as villages, cities, wards, and minor civil divisions (MCDs). This approach permitted the convenient aggregation of EDs into larger geographic entities for tabulation and publication. It also underlined the need for maps to show the boundaries of counties, incorporated places, and MCDs in their correct location. The Census Bureau began to improve the ED maps, and for the 1930 census, included ED boundaries and numbers on all maps used in the field canvass.

The ED, with minor revisions and improvements, continued to be the smallest geographic unit for which census data were available until census 11-2Census Blocks and Block Groups