How do judges draw a border across open water, and is 'equal distance' fair?

Start with the simplest rule you can think of: put the border exactly halfway. Lawyers call it the equidistance principle. An equidistance line is one where every point is equally far from the nearest points on each country's coast. It is endorsed by the UN Convention on the Law of the Sea, although it is older than that treaty.

The problem is the shape of coasts. In 1969 three neighbours took a dispute to the International Court of Justice. Beneath the North Sea lay a continental shelf rich in oil and gas. Germany's coast is concave, curving inward, while the Netherlands' and Denmark's are convex, bulging outward. A halfway line drawn from those coasts would have given Germany a smaller portion of the shelf than either neighbour. Germany argued for a share in proportion to its adjacent land, "a just and equitable share", not for equidistance.

Denmark and the Netherlands had ratified a 1958 convention that made the median line the default unless special circumstances justified another line. Germany had not. So the Court asked a sharper question: can a rule in a treaty bind a country that never joined it? It said this is possible, but "not lightly", and that it takes extensive and virtually uniform state practice together with the feeling that the rule is a legal obligation. The Court also noted that the scope and meaning of equidistance in the 1958 text remained unclear. It told the parties to abate the effect of the "incidental special feature" of Germany's concave coast, and in later negotiations Germany got most of the extra shelf it had sought.

The lesson for sea borders is that equidistance is the starting point, and fairness corrects it. Commentators call this "equity beyond the law": the judge supplements the law with equitable rules that the decision needs.