Nationality of a ship, the country in which the ship is registered. Hence, a British flag vessel is a vessel registered in the United Kingdom.

A flag of convenience is the registration of a ship in a country whose tax on the profits of trading ships is low or whose requirements concerning manning or maintenance are not stringent. This is sometimes referred to as a flag of necessity.This latter conveys the idea that trading from the shipowner’s country of nationality is too costly and that flagging out is the only way for a shipowner to continue trading the ship.