“A symbol of freedom. When a slave was manumitted by the Romans, a small Phrygian cap, usually of red felt, called pileus, was placed on his head; he was termed libertinus (‘freeman’), and his name was registered in the city tribes. When Saturninus, in 100 BC, possessed himself of the Roman Capitol, he hoisted a similar cap to the top of his spear, to indicate that all slaves who had joined his standard should be free; Marius employed the same symbol against Sulla; and when Caesar was murdered, the conspirators marched forth in a body with a cap elevated on a spear, in token of liberty. In the French Revolution, the red cap of liberty was adopted by the revolutionists as an emblem of their freedom from royal authority.”
Excerpted from: Murphy, Bruce, ed. Benet’s Reader’s Encyclopedia, Fourth Edition. New York: Harper Collins, 1996.