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,
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.