By the 15th century, the symbol had developed into its familiar form. Since at least the 15th century it has been used in military flags or insignia, typically expressing the recklessness or ferocity of the unit displaying it, and has additionally become associated with piracy in the 18th century. It came to be used specifically to mark the entrances of Spanish cemeteries, and has been used to mark poison and other dangerous liquids and powders since the 19th century.