International frontiers and boundaries separate land, rivers and lakes subject to different sovereignties. Frontiers are zones of varying widths and they were common many centuries ago. By 1900 frontiers had almost disappeared and had been replaced by boundaries that are lines. The divisive nature of frontiers and boundaries has formed the focus of inter-disciplinary studies by economists, geographers, historians, lawyers and political scientists. Scholars from these disciplines have produced a rich literature dealing with frontiers and boundaries. The authors surveyed this extensive...