The skull-and-crossbones flag of a pirate ship. It was more than a declaration of the ship's allegiances to plunder and booty--it was an invitation to ships to surrender, with the implication that those who surrendered would be treated well. In fact, few pirates killed or hurt anyone who voluntarily gave up their money, and private passengers and sailors were usually not robbed of their private possessions.

However, if a ship resisted and the pirates raised a red flag, that meant that the pirates would give no quarter and would seek to kill anyone who did not surrender unconditionally. Any prisoners could be held for ransom, cast adrift, marooned, tortured, or enslaved. That way, word got around--if you surrender to the pirates, you'll lose nothing but your gold. If you fight, you're in big trouble.

Research: GURPS Swashbucklers, Third Edition, by Steffan O'Sullivan, (C) 1999 by Steve Jackson Games, pp. 66-67, 71.