Tip: Use HistoryGeo To Track Changing U. S. County Boundaries

Brighton, Colorado (now in Adams County). Note:  I clicked on a ramdom spot near the center of Brighton.

Mary L. (Burnight) Lytle (1853-1883), my 1st cousin 5x removed, died in Brighton, Arapahoe, Colorado.  By 1910 her daughter Isabel was living in Brighton, Adams, Colorado, same area. 

There are different ways to determine what county a particular place was in, and one that I've found to be very helpful is HistoryGeo, a subscription based website that takes Bureau of Land Management land records and maps them, called the First Landowner's Project:

Here, you will find our ongoing effort to accurately map the original landowners of the 29 U.S. public land states and Texas.

This limits the time period between about 1810 to 1940, and doesn't include the land in the eastern part of the U.S. that was measured by metes and bounds, but if you are trying to determine a place in that time period and in the more western areas you might find this helpful. 

U. S. County Boundary History, from 1810 through 1940.  Note that the info is from IPUMS, which is a sophisticated data tool that I have yet to really figure out lol.

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