Public international law governs the relations of nations. The sources of public international law are very different from the sources of U.S. law (also called domestic or national law). Article 38 of the Statute of the International Court of Justice (ICJ), which is an appendix to the UN Charter, lists the sources of law that the ICJ may consider. These sources are, therefore, what all bodies within the international legal system will look to when deciding international law matters. They are as follows:
Treaties, which are written agreements between states, are at the top of the list. After treaties, custom is listed as a source of international law. Customary law is the widespread repetition (state practice) of similar international acts over time. The acts must occur out of sense of obligation (“opinio juris”). Customary law can be important as nations, may be bound by it even if they are not parties to a particular treaty.
Note that, unlike treaties, custom is not written down. One must look for evidence of custom in other sources such as cases, writings by international law scholars (i.e. treatises), IGO (intergovernmental organizations) documentation and treaties themselves. Today IGOs and their documentation are quite prevalent in international law and in international human rights law, in particular. The documentation of these bodies is often called “soft law”, as it is not binding, but it can be important as evidence of custom. Yet, Article 38 does not mention the documentation of IGOs. Why? Perhaps IGO documentation is not mentioned because the Statute of the International Court was adopted back in 1945 as an appendix to the U.N. Charter. At that point in time, there were not nearly as many IGOs as we see today. Like custom, general principles are not written down. They are derived by analogy from rules common to many legal systems. These principles come from the cases decided by national courts throughout the world.
The secondary sources of law in the international legal system resemble those in the civil law system. The chart below is useful for comparison purposes.
from Marci B. Hoffman & Robert C. Berring, International and Foreign Legal Research: a coursebook Leiden: Martinus Nijhoff Publishers, 2008. K85 .H64 2008 Reserve and 2nd ed. 2012 ebook