From what is customary international law derived?

Customary international law is derived primarily from the common practices of nations that are accepted as legal obligations. These practices develop over time and become recognized as customary law when they are consistently followed by states out of a sense of legal duty. This is characterized by two main elements: state practice, which refers to the general and consistent behavior of states, and opinio juris, which is the belief that such practices are legally obligatory.

The practice must be widespread and representative of state behavior to qualify as customary international law, and it is often seen as binding on states even in the absence of formal written agreements. This contrasts with the other options, which may contribute to international law in different ways, but do not specifically define customary law itself. Legislative acts and international agreements represent codified forms of law, while judicial decisions can interpret and apply laws but do not create them.