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 his/her duties, a judge ad hoc is required to make the same solemn declaration as an elected Member of the Court. He/she does not necessarily have to have (and often does not have) the nationality of the designating State.
A judge ad hoc takes part in any decision concerning the case on terms of complete equality with his/her colleagues and receives a fee for every day on which he/she discharges his/her 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, which occur not all that infrequently, where there are more than two parties to the dispute, it is laid down that parties which are actually acting in the same interest are restricted to a single judge ad hoc between them - or, 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 accordingly 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 from the foregoing 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, where one or more elected judges do not sit, or as many as 16 or 17 where 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 who are not 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 with respect to provisional measures, preliminary objections and the 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 judgment with respect thereto, its composition will not change.
The right of an elected judge having the nationality of one of the parties in a case to sit in the case has not been seriously questioned by legal scholars. It is clear simply from the result of the votes taken by the Court and from the separate and dissenting opinions submitted that such judges have often voted in disaccord with the submissions of 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 participating in its deliberations a person more familiar with the views of one of the parties than the elected judges may sometimes be.