A black flag was the original pirate flag, and served as a warning to surrender or face death. As time went on, some pirates began customizing the plain black flag to make it seem more fearsome. From the plain black flag came the jolly roger or skull and crossbones flag, featuring a skull with two crossed bones. Pirate flags became varied and numerous after that, but the original flag flown on corsairs about to board a boat was a plain black flag.