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Empirical researchers often have to map data provided for a" reporting" spatial unit, say counties in 1900, to a" reference" one, say, counties in 2010. We discuss a general method to create such crosswalks: computing the share of the area of each reporting unit nested in a given reference unit. Using these shares, data can be re-aggregated from the reporting to the reference units. We apply the method to construct a crosswalk for US county-level data since 1790 to present-day counties or commuting zones. We also provide the code to generate other crosswalks given maps of reporting and reference units.