E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Cap of Liberty.
When a slave was manumitted by the Romans, a small red cloth cap, called pilëus, was placed on his head. As soon as this was done, he was termed libertinus (a freedman), and his name was registered in the \??\
tribes. When Saturninus, in 263, possessed himself of the capitol, he hoisted a cap on the top of his spear, to indicate that all slaves who joined his standard should be free. When Marius incited the slaves to take up arms against Sylla, he employed the same symbol; and when Cæsar was murdered, the conspirators marched forth in a body, with a cap elevated on a spear, in token of liberty (See LIBERTY.)