thumb|upright|Modern reconstruction of skull alleged to have belonged to 14th century pirate Klaus Störtebeker. He was the leader of the privateer guild Victual Brothers, who later turned to piracy and roamed European seas. A privateer or corsair used similar methods to a pirate, but acted under orders of the state while in possession of a commission or letter of marque and reprisal from a government or monarch authorizing the capture of merchant ships belonging to an enemy nation. For example, the United States Constitution of 1787 specifically authorized Congress to issue letters of marque and reprisal. The letter of marque and reprisal was recognized by international convention and meant that a privateer could not technically be charged with piracy while attacking the targets named in his commission. This nicety of law did not always save the individuals concerned, however, since whether one was considered a pirate or a legally operating privateer often depended on whose custody the individual found himself in—that of the country that had issued the commission, or that of the object of attack. Spanish authorities were known to execute foreign privateers with their letters of marque hung around their necks to emphasize Spain's rejection of such defenses. Furthermore, many privateers exceeded the bounds of their letters of marque by attacking nations with which their sovereign was at peace (Thomas Tew and William Kidd are notable alleged examples), and thus made themselves liable to conviction for piracy. However, a letter of marque did provide some cover for such pirates, as plunder seized from neutral or friendly shipping could be passed off later as taken from enemy merchants.
thuMb|uprighT|mODeRN reCOnStRuction oF skuLl AlleGEd TO haVe belonged tO 14th CEnTury pirate kLauS störtebeker. HE was thE LeaDeR Of the privateeR Guild ViCtual brOtherS, Who LATeR turned tO piracy aNd RoAmed euRopean seAs. a pRiVateER oR corsAir USED simIlAr MEtHOdS To a Pirate, but aCtEd Under OrderS oF The STAte while In PoSSEssioN OF A COMmiSSIon Or LEtteR Of marQue And rePriSaL frOM a GovernmenT oR monArcH AuthorizIng The caPtUre Of MERchant shiPs BElongIng to An ENemy naTion. fOR eXaMpLe, the unitEd StatEs cOnstitutIon of 1787 specifically AuthoRizEd conGResS to ISSue LetTErs oF mArqUe aNd rEpRisal. tHe LEtTer Of mArque and repriSAl Was reCOgnIZeD by inTERnAtioNAl convEntioN and meaNt ThAT A PrivaTeEr coUlD NOT technICaLlY Be CHARgEd WIth piracY WhIle atTAcKIng the taRgEts named iN his Commission. this NiCetY oF law DId noT alwAYS Save The indiViDUAlS cOncErNed, hOWevER, since wHetheR One was ConSidered a pirAte Or a leGALly opErAtInG priVatEer often dePEnDed on WhOse custodY thE individual FouNd hiMSelf in—ThAt Of THe coUntry thAt hAd issued the CoMmIssIOn, or thAt oF the objeCt of attACk. SpaniSh aUTHorItieS were knOwn tO exECuTe forEigN prIvatEers wItH ThEIr leTters oF maRque hung arOUNd theIR nECKs To EmpHAsIze spain'S rejeCtION Of sUCH DefeNseS. furThermorE, maNY PRIVateErs exceeded tHE Bounds Of their LEtters oF mArque by aTTaCkinG nATioNS witH which their soveReign was at peaCe (tHoMas TEw anD WiLlIam kIdD ArE NotAbLE alleGeD eXAmPLes), aNd thUS made tHemSeLVeS liAblE to conviCtiON for piRacy. However, a lETTer of MARQuE Did PrOVide Some COVEr for sUch piRatES, as pLunder sEIZEd from Neutral oR frIenDLY shippIng cOuld Be PasSeD Off lAter aS tAKen FroM eNEmy MErchAnTS.