The boundary dataset contains measurements of all international boundary lengths between members of the international system between 1815 and 1997. The dataset, being based on the Correlates of War system membership list, provides exact dates of all changes in boundaries throughout the period and explanatory notes related to these changes and the parties involved. Moreover, the documentation accompanying the data offers a comparison between the boundary length data and alternative contiguity datasets. The data are available in a ready-to-use dyadic format (Stata) and as a comprehensive (Excel) file with notes and comments on specific events.