If you know the state, county, town/city, and street name, the maps at 1940 U.S. Federal Census (http://www.1940census.net/1940_enumeration_district_maps.php) will help you find the enumeration district (E.D.). With the E.D. at hand, you will be able to browse the census images which will be available on April 2, 2012.
--->Don't confuse 1940 U.S. Federal Census with the 1940 U.S. Census Community Project. The two are completely different websites.<---
My parents lived most of their married lives in Mineral Ridge, Trumbull County, Ohio. I just recently learned that the little village of Mineral Ridge crossed the boundaries of two counties: Trumbull and Mahoning. I'm also in the situation of having parents who married in 1939. I know they lived in Niles and in Warren after they married but I don't know when they bought their home in Mineral Ridge. If they're not in the Ridge in 1940, I'll be looking for them in the two other cities.
Below, on the left is Trumbull County, on the right is Mahoning County. You can see the outline of the Ridge in both. Although in two different counties, the E. D. is the same for the whole village.
Not all census maps are created equal. The one for Mineral Ridge has only the main thoroughfares named. It's unimportant since the village of Mineral Ridge is all in the same E.D. On the maps of large towns and cities which have many enumeration districts the street names appear. If your ancestor lived on a named street in a large town or city you'll probably be able to find it on a map and identify the E.D.
Please note that after you click "Get ED Map Images" the next screen that opens may give you a list of two or more maps to view. You may need to look at more than one to find the street you're searching for.
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When the 1940 Census becomes available on April 2, it will not be indexed. If we have no idea where our ancestors lived, we'll have little hope of finding them until the census is indexed. I encourage you to help index at the 1940 U.S. Census Community Project. You'll help yourself and help others, too.