The cervelliere is a round, close-fitting steel skull cap. It was worn as a helmet

Helmet

A helmet is a form of protective gear worn on the head to protect it from injuries.Ceremonial or symbolic helmets without protective function are sometimes used. The oldest known use of helmets was by Assyrian soldiers in 900BC, who wore thick leather or bronze helmets to protect the head from...

 during the medieval period.

History

It was first introduced during the late 12th and was the primary head protection for Crusaders at that time and was worn either alone or over chainmail

Chainmail

Mail is a type of armour consisting of small metal rings linked together in a pattern to form a mesh.-History:Mail was a highly successful type of armour and was used by nearly every metalworking culture....

. By the late 13th century a great helm

Great helm

The great helm or heaume, also called pot helm, bucket helm and barrel helm, of the High Middle Ages arose in the late twelfth century in the context of the crusades and remained in use until the fourteenth century...

 would usually be worn over a cervelliere.The back of the helmet grew longer over time and evolved into the bascinet

Bascinet

The bascinet was a Medieval European open-faced military helmet, typically fitted with an aventail and hinged visor. The term is also written as bassinet or basinet.-Early versions:...

. Cerveillieres were worn throughout the medieval period and even during the Renaissance. Cheap and easy to produce, they were much used by commoners and non-professional soldiers who could not afford more advanced protection.