A document given to a sailor (privateer) giving him amnesty from piracy laws as long as the ships plunders are of an enemy nation. A large portion of the pirates begin as privateers with this symbol of legitimacy. The earnings of a privateer are significantly better than any of a soldier at sea. Letters of marque aren't always honored, however, even by the government that issues them. Captain Kidd had letters of marque and his own country hanged him anyway.

Related: Famous Pirates