In this undated photo provided by the Franklin D. Roosevelt Presidential Library and Museum, staffers at the U.S. Census Bureau in Washington mark up a map for use in the 1940 census. The map shows approximately 147,000 enumeration districts for census-taking purposes and will ensure against overlapping activities of census takers, or enumerators, and also to avoid missing any territory. (AP Photo/Franklin D. Roosevelt Presidential Library)
The 1940 census records were released Apr. 2, with more than 21 million people alive in the U.S. and Puerto Rico.