I learned a very valuable family history research principle years ago, and I’ve seen its application more and more as I gain experience in genealogical research. It is a powerful tool for breaking through some brick walls.
Like ourselves, our ancestors did not neatly compartmentalize themselves into cities and counties. They might live here, own a farm in the next county, and have cousins down the road across the state line. Despite any geographical proximity, if you cross a city, county, state, or any other boundary, that’s a different jurisdiction – and often an entirely different database.
It is very quick and easy to look on the internet to see what your ancestor’s surrounding counties are. Just Google something like “Tennessee county map,” and look at a map it returns to see what the surrounding counties are. If the county lies on a state line, you should see what counties it borders in the neighboring state as well. For counties in the United States, an easy way to do this is to look up the county in Wikipedia. At the bottom of the article page, you can usually find a box with links to all the adjacent counties – even those across state lines. This will even show you jurisdictions across the Mexican and Canadian borders. Very handy!
So, before you decide your Detroit ancestor dropped off the face of the earth for the 1910 Census, remember that you might consider looking in Ontario in the 1911 Canadian Census instead.