A red flag signaled that no quarter would be given, meaning the pirates would kill everyone on board. The standard black flag offered victims a chance to surrender safely.

This tradition began in World War I after an admiral claimed submariners should be hanged like pirates. Crews mockingly adopted the flag to tally their sunken enemy vessels.

By displaying intimidating symbols like skulls, pirates instilled intense terror in merchant crews. This tactic often secured wealthy cargoes without risking a costly battle.

The term likely evolved from the French phrase "joli rouge," meaning pretty red. English sailors gradually anglicized this name when referring to pirate flags of any color.