The cap of a crown is the cap which fills the inner space of a modern crown. While ancient crowns contained no cap, from mediaeval times it became traditional to fill the circlet with a cap of velvet or other such cloth, with a base of ermine.[1]
Notes and References
Book: Norris, Herbert . Medieval Costume and Fashion . 1 January 1999 . Courier Corporation . 978-0-486-40486-8 . 177 . en.
