The tall, cylindrical helmet worn by many European knights in the 12th and 13th centuries was called the great helm.
Great helms enclosed the entire head and usually had a flat or slightly rounded top, narrow eye slits, and ventilation holes. They developed from earlier nasal helmets and offered stronger protection for the face during mounted combat.
Knights often wore a padded cap or mail coif beneath the metal helmet. Because a great helm was heavy and restricted vision and breathing, it was commonly used for battle or tournaments rather than continuous daily wear.
The great helm should not be confused with the bascinet, which had a rounded skull and became especially common in the 14th century. Later helmets, including the armet, provided improved mobility and a closer fit around the head and neck.