 This lesson is about customer international law, that is the second source of international law listed by article 38 paragraph 1 of the ICJ Statute. Let me explain why I start the study of the sources of international law with customer international law rather than with treaties and conventions which are mentioned first in article 38. There are two reasons for this. First, it is because to a large extent the making of treaties and conventions is itself regulated by rules of customer international law. Therefore, it is important to understand what is customer international law before turning to treaties. Second, it is because customer international law is, except in certain cases that will be mentioned, it is general in scope international custom is as a matter of principle binding on all states and also on international organizations. So customer international law is also called general international law. And this stands in contrast to treaties, because treaties as treaties are only binding on their contracting parties, that is on the states or the international organizations that have explicitly consented to them. And also in contrast to treaties, which are written documents, custom is as such not contained in any specific legal document, both in domestic law and in international law, custom floats in the air as it were. It is there, but it is nowhere. We say it exists, but how do we know it exists when it is not the result of the exchange of agreements over a written text? Of course, one may decide to write down custom, to make it explicit in a document using words and sentences, but writing down custom would not make custom, it would simply make it explicit while it was there and nowhere before being written down. So in this lesson about customer international law, we first, the first thing to understand is to try to understand how the formation of rules of customer international law is generally explained. And you see how careful I am. I'm not saying that we're going to study the process by which customer rules are created. I said we'll try to understand the explanations that are usually given in order to rationalize that process. And that issue remains hotly debated and largely mysterious, because customer is fundamentally the result of an informal, unregulated, spontaneous and decentralized process. And it cannot be otherwise. At the same time, everybody intuitively understands what is custom. Custom is something that is usually done and that should be done. The explanation that is generally provided in international law is not much different. And indeed, the International Court of Justice has developed what has been called the theory of the two elements which, when they meet, result in the making of rules that can be said to exist as rules of customer international law. The two elements are expressed by article 38, international custom as evidence of a general practice accepted as law, two elements, general practice, and general practice accepted as law. Let us turn now to some ICJ judgments in order to see the theory of the two elements at play.