Under Article 31, paragraphs 2 and 3, of the Statute of the Court, a State party to a case before the International Court of Justice which does not have a judge of its nationality on the Bench may choose a person to sit as judge ad hoc in that specific case under the conditions laid down in Articles 35 to 37 of the Rules of Court. Before taking up their duties, a judges ad hoc are required to make the same solemn declaration as an elected Member of the Court. They do not necessarily have to be (and often does are not) nationals of the State that designates them.
Judges ad hoc take part in any decision concerning the case on terms of complete equality with their colleagues and receive a fee for every day on which they discharge their duties, that is to say, every day spent in The Hague in order to take part in the Court’s work, plus each day devoted to consideration of the case outside The Hague.
A party must announce as soon as possible its intention to choose a judge ad hoc. In cases where there are more than two parties to the dispute, which is a not infrequent occurrence, the rule is that parties acting in the same interest are restricted to a single judge ad hoc between them and that, if one of them already has a judge of its nationality on the Bench, they are not entitled to choose a judge ad hoc at all. There are therefore various possibilities, the following of which have actually occurred in practice: two regular judges having the nationality of the parties; two judges ad hoc; a regular judge of the nationality of one of the parties and a judge ad hoc; neither a regular judge having the nationality of one of the parties nor a judge ad hoc.
It follows that the composition of the International Court of Justice will vary from one case to another, and that the number of judges sitting in a given case will not necessarily be 15. There may be fewer, if one or more elected judges do not sit, or as many as 16 or 17 if there are judges ad hoc; in theory there may even be more than 17 judges on the Bench if there are several parties to a case which are not acting in the same interest. The composition of the Court may also sometimes vary from one phase of a case to another: in other words, the composition need not necessarily be the same for provisional provisional measures, preliminary objections and merits. Nevertheless, once the Court has been finally constituted for a given phase of a case, i.e., from the opening of the oral proceedings on that phase until the delivery of the judgment relating to it, its composition will not change.
Legal scholars have not seriously questioned the right of an elected judge to sit in a case when he/she has the nationality of one of the parties. It is evident from the result of the votes taken by the Court and the judges' separate and dissenting opinions that such judges have often voted against the submissions put forward by their own country. The institution of the judge ad hoc, on the other hand, has not received unanimous support. Whilst the Inter‑Allied Committee of 1943 argued that “[c]ountries will not in fact feel full confidence in the decision of the Court in a case in which they are concerned if the Court includes no judge of their own nationality, particularly if it includes a judge of the nationality of the other party”, certain members of the Sixth Committee of the General Assembly of the United Nations expressed the view, during the discussions between 1970 and 1974 on the role of the Court, “that the institution, which was a survival of the old arbitral procedures, was justified only by the novel character of the international judicial jurisdiction and would no doubt disappear as such jurisdiction became more firmly established”. Nevertheless, numerous writers take the view that it is useful for the Court to have a judge participating in its deliberations who is more familiar with the views of one of the parties than the elected judges may sometimes be.