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When searching for family of slave descendants; remember that slaves often were not on traditional records. Often, their geneology may be traced through 'Property' records.
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Slave were not regarded as citizens/people...that, you must not forget...like wagons and horses were considered property.
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Depending on where your great-great-grandfather was born...should help to determine which plantation he may have been born on or sold to. Looking into the city/county(or parrish)/state they were born...might be reflected in State archives...maybe even in transfer of property documents in local archives.
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Also, as many decendents took as their name the Master's family...some also took on the 'town or river or familiar' name...especially after the Civil War.
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