Custom includes all the norms of general international law that bind all States of the international community. An international custom describes a constant and unchanging behavior hold by States: States repeat and follow a customary behavior because they are convinced of its necessity. Therefore, two main elements characterize custom:
1) Diuturnitas: It is the constant repetition of a specific behavior through time by a significant number of actors.
2) Opinio juris sive necessitatis: It is the idea that a specific behavior is juridical mandatory or that it is necessary that it will be so.
As for the time for custom to consolidate, it will be shorter the more widespread the customary behavior amongst the international community is. Indeed, at least four sources produce customary law:
1) The outer acts of States: a) treaties; b) diplomatic notes; c) several behaviors within international organizations.
2)The inner acts of States: a) laws; b) sentences; c) administrative acts.
3)The internal jurisprudence.
4)The jurisprudence of international supreme courts.
Custom creates new general international law that binds all States, whether they did or did not participate to the birth of it. However, there are States that may deny the applicability of a custom that they did not contribute to create. The dispute may repeatedly originate 1) by a single State that will hence bear the name of “persistent objector”, but it is irrelevant and equally binding the objector State, or 2) by a group of States, and in this case it is difficult to ignore, although if the majority of States does not consider a behavior as binding then in fact it cannot be considered a custom at all.
Alongside with international custom, smaller and more specific customs exist that bind a small circle of States (regional or local customs). Special customs may link geographically neighboring or close States, contracting parts of a treaty, contracting parts of a bilateral agreement, etc.
During the past years, the international community has increasingly felt the need to codify custom. The codification of custom (that is the codification of general international law) was primarily promoted by special codification conventions of the United Nations. The origin of the codification of customary general international law dates back to the end of the nineteenth century. For instance, the two Hague Conventions of 1899 and 1907 attempted to codify the international law of war. Afterwards, from 1919 onwards, the League of Nations tried to codify several aspects of international law, although without a great success. Things changed with the birth of the United Nations in 1945. Art 13 of the UN Charter provides that the UN General Assembly can undertake studies and propose recommendations in order to encourage the progressive development of international law and its codification. Indeed, a 1947 General Assembly Resolution established a subsidiary body named International Law Commission (ILC). Located in Geneva, the ILC has the task to prepare codification texts for customary norms on several subjects. Generally, it arranges in advance the codification projects discussed in subsequent international multilateral conventions. It often happens that the UN General Assembly or other solemn conference of States adopt these projects, opening them to the ratification by States. The ILC predisposed many conventions of codifications, the majority of which have been ratified by a great number of States. Some of the main ILC conventions are the following:
1) The Vienna Convention on Diplomatic Relations (1961);
2) The Vienna Convention on Consular Relations (1963);
3) The New York Convention on Special Missions (1969);
4) The four Geneva Conventions on the Law of the Sea (1958);
5) The Vienna Convention on the Law of Treaties (1969);
6) The Vienna Convention on the Law of Treaties between States and International Organizations or between International Organizations (1986);
7) The Vienna Convention on Succession of States in Respect of Treaties (1978);
8)The Vienna Convention on Succession of States in Respect of State Property, Archives and Debts (1983);
9)The New York Convention on the Law of the Non-Navigational Uses of International Watercourses (1997);
10)The New York Convention on Jurisdictional Immunities of States and Their Property (2004);
11) The Montego Bay Convention on the Law of the Sea (1982).
Besides the ILC, the UN General Assembly summoned some other special conferences of States to codify customary law: think of the Conference of Rome that in 1998 approved the Statute of the International Criminal Court.
As known, the codification agreements, alike all other international agreements, only bind the contracting parts and thus apply just to States that ratify them. All UN agreements are stipulated for an endless period and only few of them contemplate revision procedures for their norms (e.g. the Geneva Conventions of 1958).
Finally, private codifications of international law exist, like those promoted by the Institut de Droit International (IDI) located in Geneva, which adopts codification projects once approved by its plenary assembly.
A. Cassese, International Law, USA, Oxford University Press, 2001.