Quarantine "yellow jack" but this was also a name for yellow fever, which probably derives its common name from the flag, not the color of the victims (cholera ships also used a yellow flag). The plain yellow flag ("Quebec" or Q in international maritime signal flags) probably derives its letter symbol for its initial use in "quarantine", but this flag in modern times indicates the opposite—a ship that declares itself free of quarantinable disease, and requests boarding and routine port inspection. Australia has perhaps the world's strictest quarantine standards. Quarantine in northern Australia is important because of its proximity to South-east
Flag of convenience enforcement by countries when they visit ports in those countries, called port state control. International law requires that every merchant ship be registered in a country. The country in which a ship is registered is its flag state, and the flag state gives the ship the right to fly its civil ensign. A ship operates under the laws of its flag state, and these laws are used if the ship is involved in an admiralty case. 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