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.
While the precise reason for the inclusion of a cap is unknown, two reasons are often given:
- to add to the visual impact of the crown, while showing off the golden circlet to maximum effect;
- to keep a monarch's head warm in drafty mediaeval buildings during long coronation ceremonies or public events where crowns were worn.