The Phrygian or similar cap, a symbol of freed slaves and thus of liberty
The Phrygian cap or liberty cap is a soft conical cap with the top pulled forward, associated in antiquity with several peoples in Eastern Europe and Anatolia, including Phrygia, Dacia and the Balkans. In early modern Europe it came to signify freedom and the pursuit of liberty through a confusion with the pileus, the felt cap of manumitted (emancipated) slaves of ancient Rome. In artistic representations it signifies freedom and the pursuit of liberty.
The introductory paragraph uses material from the Wikipedia article "Phrygian cap" as of 22 Oct 2018, which is released under the Creative Commons Attribution-Share-Alike License 3.0.