The great helm was the medieval European helmet that enclosed a knight’s entire head and face.
It developed in the late 12th century as helmets increasingly protected the face and sides of the head. Great helms were typically made from several iron or steel plates and had narrow eye slits and ventilation holes. Their flat-topped or cylindrical shapes are strongly associated with the high and late Middle Ages.
The helmet offered substantial protection in mounted combat, especially during a lance charge. However, it was heavy, hot, and restricted vision and breathing. For that reason, knights often wore it over a lighter mail coif or another helmet and removed it when not directly fighting.
The great helm is sometimes confused with the later bascinet. The bascinet generally offered a more adaptable skull shape and could have a movable visor, while the great helm formed a more enclosed outer shell.