Pileus
From NovaRoma
The pileus (Latin, plural, pilei) — also pilleus or pilleum — was a cap worn by sailors in Ancient Greece (where it was the pilos or pilidion) and later copied by Ancient Rome. It was a brimless, felt cap, somewhat similar to a fez. The pilleolus was a smaller version, similar to a skullcap.
The pileus was especially associated with the manumission of slaves who wore it upon their liberation. It became emblematic of liberty and freedom from bondage. During the classic revival of the 18th and 19th centuries it was widely confused with the Phrygian cap which, in turn, appeared frequently on statuary and heraldic devices as a "liberty cap."
