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Historically, acquisition of new territory and conversion of that territory into organized states happened very quickly. Between 1790 and 1920, each decade saw changes in the territorial extent or the make-up of states in the country. This period of change was followed by several decades during which there were no changes until Alaska and Hawaii were admitted as states.

SOURCE: Decennial censuses 1790 to 1960



NOTE: The territorial extents shown reflect the composition of the U.S. at the time of each decennial census and are based on the date by which purchase, treaty ratification, or act of the U.S. Congress occurred. In addition to decennial census publications, the “Map Guide to the U.S. Federal Censuses, 1790-1920” by William Thorndale and William Dollarhide (Genealogical Publishing Com, 1987) was also used for reference. For cases in which the U.S. and another country laid claim to the same territory, the extent was included or excluded from those shown in this map series based on whether the area had been included in the U.S. decennial census for a given decade. Current and historical information about the Outlying Areas and the Island Areas, including Puerto Rico, can be seen at www.census.gov/history/www/programs/geography/island_areas.html.