Genealogical research in U.S. censuses begins with identifying correct county jurisdictions ??o assist in this identification, the map Guide shows all U.S. county boundaries from 1790 to 1920. On each of the nearly 400 maps the old county lines are superimposed over the modern ones to highlight the boundary changes at ten-year intervals. Accompanying each map are explanations of boundary changes, notes about the census, & tocality finding keys. In addition, there are inset maps which clarify ??erritorial lines, a state-by-state bibliography of sources, & an appendix outlining pitfalls in mapping county boundaries. Finally, there is an index which lists all present day counties, plus nearly all defunct counties or counties later renamed-the most complete list of American counties ever published.