The Phrygian or similar cap, a symbol of freed slaves and thus of liberty

Reference

Liberty cap - Wikipedia, the free encyclopedia
"A liberty cap is a brimless felt cap, such as the Phrygian cap or pileus, emblematic of manumission in the Ancient World and Liberty in modern revolutionary periods. ..."
Phrygian cap - Wikipedia, the free encyclopedia
"The Phrygian cap is a soft, red, conical cap with the top pulled forward, worn in antiquity by the inhabitants of Phrygia, a region of central Anatolia. In paintings and caricatures it represents freedom and the pursuit of liberty. ..."
Pileus (hat) - Wikipedia, the free encyclopedia
"The pileus (plural, pilei), also pilleus or pilleum, was, in Ancient Greece and Rome, a brimless, felt cap, somewhat similar to a fez. (The pilleolus was a smaller cap, similar to a skullcap.) It was especially associated with the manumission of slaves who wore it upon their liberation. The pileus became emblematic, especially popular in the 18th and 19th Centuries (when it was often called a 'liberty cap' or Phrygian cap), of liberty and freedom from bondage, appearing on statuary and on heraldic devices. ..."