[1] International law requires that every ship be registered in a country, called its flag state.
A ship's flag state exercises regulatory control over the vessel and is required to inspect it regularly, certify the ship's equipment and crew, and issue safety and pollution prevention documents.
In some cases, such as the United States' Alternative Compliance Program, the registry can assign a third party to administer inspections.
The Liberian Registry, for example, registers seagoing vessels of more than 500 net tons that conduct foreign trade.
The principle was repeated in Article 91 of the United Nations Convention on the Law of the Sea of 1982 (UNCLOS), which came into effect in 1994.
[9] To come into force, the 1986 treaty requires 40 signatories whose combined tonnage exceeds 25% of the world total.
Open registries do not have such requirements; some offer on-line registration, and one guaranteed completion in less than a day.