Treaties are the means states use to spell out the terms of their relations on a particular issue. A tr eaty is similar to a legal contract or agreement, such as a lease or rental agreement for an apartment. States consent to the obligations that treaties spell out. As a source of international law, treaties specify international rules of conduct.

Not all treaties constitute a general or universal rule of international law. For instance, the North American Free Trade Agreement (NAFTA) between the US, Canada, and Mexico is law that concerns only those three countries, even though NAFTA does affect other states.

Treaties can be multilateral or bilateral.

In the absence of a treaty, states sometimes follow a consistent practice over a period of time and come to accept this practice as a binding legal obligation. Over time, other states adopt the practice because of its usefulness, and it becomes a general practice with a legal obligation attached to it. States who fail to follow this custom risk punishment, sanctions, and retaliation. The practice then has become part of customary international law (CIL).

Typical American sailing vessel, circa 1870

For example, during the nineteenth century, Britain required its seagoing vessels to display colored lights to avoid collisions. Britain’s requirement eventually became international law. The Scotia case involved a collision between the American sailing vessel Berkshire and the British steamer Scotia. In 1872, the US Supreme Court ruled that the Berkshire had failed to display colored lights in accordance with British rule, which by then had become the required practice of other states and was, therefore, the “law of nations”.

Typical British steamer, circa 1870

In another case, the Paquette Habana case in 1900, the US Supreme Court recognized that customary international law protects fishing boats from capture by belligerents during warfare.

Most of the international laws governing the freedom of the high seas, privileges and immunities of states, jurisdiction over territory, and the rights of aliens emerged first as customs before they be came laws.