The Jolly Roger is any of various flags flown to identify a ship's crew as pirates that were about to attack. The flag most commonly identified as the Jolly Roger today is the skull and crossbones (although swords are also common), a flag consisting of a human skull above two long bones (probably tibias) set in an x-mark arrangement, most usually depicted crossing each other directly under the skull, on a black field. This design was used by several pirates, including Captains "Black Sam" Bellamy, Edward England, and John Taylor. Some Jolly Roger flags also include an hourglass, another common symbol representing death in 17th- and 18th-century Europe. Despite the prominence of flags with art in popular culture, plain black flags with no art were often employed by most pirates in the 17th㥾th century. Historically, the flag was flown to frighten pirates' victims into surrendering without a fight, since it conveyed the message that the attackers were outlaws who would not consider themselves bound by the usual rules of engagementand might, therefore, slaughter those they defeated (since captured pirates were usually hanged, they did not have much to gain by asking quarter if defeated). The same message was sometimes conveyed by a red flag, as discussed below.
Since the decline of piracy, various military units have used the Jolly Roger, usually in skull-and-crossbones design, as a unit identification insignia or a victory flag to ascribe to themselves the proverbial ferocity and toughness of pirates. In a non-naval context the skull and crossbones motif has additional meanings, for example, to signify a hazard such as poison.