Crossbones are two bones placed in an X shape, one crossing over the other. You've probably seen them on pirate flags, paired with a skull: the famous Jolly Roger that pirate ships flew to strike fear into other vessels they encountered on the high seas. That skull and crossbones symbol told everyone: “We're dangerous, and we don't follow the rules.”

Today, you'll still see crossbones as a warning symbol on labels for poisonous substances like cleaning chemicals or pesticides. The image works because bones represent death, and crossing them creates a stark, attention-grabbing design that says “danger” without using any words. It's a symbol that transcends language barriers.

The crossbones symbol actually predates piracy by centuries. Medieval tombstones and church carvings used skulls and crossbones to remind people that death comes to everyone eventually. Pirates simply borrowed this memento mori (Latin for “remember you will die”) and turned it into their battle flag, adding their own threatening twist to an ancient symbol.

While crossbones usually appear with a skull, the word is also used by itself to describe that X-shaped arrangement of bones, whether in an old cemetery carving, a warning label, or a Halloween decoration.