Acquiescence and mutual tolerance became the basis of the ICJ decision in the Anglo-Norwegian Fisheries case (UK v. Norway, 1951). The Court accepted the legality of Norway’s straight baseline system for delimiting its “Fishery Zone” by a 1935 decree. This decree was based on Norway’s earlier decrees of 1869 and 1889. No State had protested against them including the United Kingdom. The Court observed that there is a general tolerance with regard to the Norwegian practice.

Estoppel [from Norman French estouper, to stop up]: A rule of evidence or a rule of law that prevents a person from denying the truth of a statement he has made or from denying facts that he has alleged to exist. The denial must have been acted upon (probably to his disadvantage) by the person who wishes to take advantage of the estoppel or his position must have been altered as a result. There are several varieties of estoppel.

Opinio juris sive necessitatis means “an opinion of law or necessity.” It is the belief that an action was carried out because it was a legal obligation. This term is frequently used in legal proceedings as a defense in a case.

The terms treaty, pact, charter, constitution, statute, convention, General Act, Final Act (when it is an independent instrument) are generally used for formal instruments of multilateral character. The terms such as memorandum of understanding, procés vérbal, arrangement, exchange of notes are used for less formal instruments and generally related to transactions of provisional or temporary nature.