The Jolly Roger is any of various flags flown to identify a ship's crew as pirates. The flag most commonly identified as the Jolly Roger today is the skull and crossbones, a flag consisting of a human skull above two long bones (probably femurs) set in an x-mark arrangement on a black field. This design was used by several pirates, including Captains 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 its prominence in popular culture, plain black flags were often employed by most pirates in the 17th-18th 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 engagement—and might, therefore, slaughter those they defeated (since captured pirates were usually hanged, they did not have much to gain by asking quarter if defeated). Source: Wikipedia: Jolly Roger.