To determine where we should look for records, we need to know the county and state our ancestor lived. As we search, we need to review histories and maps of the county and state to see when and how the boundaries changed.
States changed their boundaries more than we think. For example, Kentucky settlements were a part of Virginia until Kentucky became a state in 1792, but continued to have boundary disputes with Tennessee until 1820. The northwestern portion of Virginia split in 1863, forming West Virginia. Colonial Louisiana included sections of ten other states, including Minnesota.
County boundaries changed more frequently than state boundaries. Present day Indian River County, Florida has changed county boundaries six times since being Indian Lands. Sections of Mellette County, South Dakota were formerly part of Cheyenne and Jackson Counties.
To begin determining changes in county and state boundaries, you can search the following books: