The Letter of Marque
Patrick Harris -A Letter of Marque was a document authorizing a mariner to prey upon enemy shipping in time of war (and sometimes at peace). It was, in effect, a legalization of piracy.
Letters of Marque date back to 1243, when the first "Letter of Marque and Reprisal" was issued by King Henry the Third of England. These licences to commit piracy were given out to specific individuals on the condition that profits be split with the crown. This was later expanded to include any reprisal (land or sea, man or good) which could be meted out upon any nation that had been perceived to wrong the mother country. While these "privateers" (as they came to be known) did not always act with the explicate permission of the reigning monarch, the king was happy as long as he got his slice of the pie.
From this splitting of profits between the crown and the ship came the Royal Navy's prize law, decided upon by the Prize Courts. Each man aboard the ship received an amount proportionate to his rank. This system would remain largely unchanged until the mid nineteenth century.
Not only was a Letter of Marque a license to commit piracy, it also made the privateers into a quasi naval force. This went to such lengths as to have commissioned privateers become protected by the rules of war. Privateers were not warships in any way though, as they single-mindedly searched for profit rather than attempting to destroy the enemy fleet.