The next point to be noted is that, regarding some relevant circumstances,
there is a degree of difference between the case law and State practice.
Consequently, special rules, which do not necessarily rely on State practice,
have developed in the framework of case law. A typical example is
the concept of proportionality. As discussed already, the large role of proportionality
as an operational rule or as a test of equitableness cannot be
explained from the viewpoints of State practice and opinio juris. The same
is true of the effect to be given to islands. Since there is no customary law
on the subject, the effect attributable to islands was to be determined, case
by case, at the discretion of international tribunals. On this point, the
courts have developed the ‘half effect’ solution regarding offshore
islands. In State practice, however, agreements giving half effect to offshore
islands are rare. In this sense, it would seem that the courts’
solutions of giving half effect to offshore islands in the Anglo–French Continental
Shelf, Tunisia/Libya and Gulf of Maine cases are a novelty in this
field different from State practice. In sum, international courts and tribunals
have developed special rules within the framework of case law,
independently of State practice.309