The equidistance principle, or principle of equidistance, is a legal concept in maritime boundary claims that a nation's maritime boundaries should conform to a median line that is equidistant from the shores of neighboring nations. The concept was developed in the process of settling disputes in which the borders of adjacent nations were located on a contiguous continental shelf:
An equidistance line is one for which every point on the line is equidistant from the nearest points on the baselines being used. The equidistance principle is a methodology that has been endorsed by the United Nations Convention on the Law of the Sea but predates that treaty and has been used by the Supreme Court of the United States, states, and nations to establish boundaries equitably.[1]
The equidistance principle represents one aspect of customary international law, but its importance is evaluated in light of other factors[2] such as history:
"Historic rights" or titles of some or another kind will acquire enhanced, rather than diminished, importance as a result of the narrowing of the 'physical' rather than the 'legal' sources of right. It is important to remember that, although historical claims were not successful in the Gulf of Maine case, the identification of a 'status quo' or 'modus vivendi' line in Tunisia–Libya was of decisive importance in confirming the equitableness of the first stage of delimitation. States will scrupulously avoid, more than ever, any appearance of acquiescence where acquiescence is not intended; prudent coordination can be expected between petroleum and mining ministries and the legal advisers of foreign ministries." — Highet, Keith. (1989). "Whatever became of natural prolongation."