The Jolly Roger is the name given to any of various flags flown to identify a ship's crew as pirates. The flag most usually identified as the Jolly Roger today is the skull and crossbones, being a flag consisting of a skull above two long bones set in an x-mark arrangement on a black field. This design was used by four pirates, captains Edward England, John Taylor, Sam Bellamy and John Martel. 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 didn't have much to gain by asking quarter if defeated).