In its simplest form, the great helm was a flat-topped cylinder of steel that completely covered the head and had only very small openings for the eyes and mouth. Later designs gained more of a curved design, particularly on the top, to deflect or lessen the impact of blows.
The great helm ultimately evolved from the nasal helmet, which had been produced in a flat-topped variant with a square profile by about 1180. From this type of helmet an intermediate type, called an ‘enclosed helmet‘ or ‘primitive great helm’, developed near the end of the 12th century. In this helmet the expansion of the nasal produced a full face-plate, pierced for sight and breathing. This helmet was largely superseded by the true great helm by c. 1240.
A later variant with a more conical top is known as a ‘sugarloaf helm’. In Spanish they are called yelmo de Zaragoza, referring to Zaragoza where they were introduced for the first time in the Iberian peninsula.
Great helms were worn with cloth and fiber padding on the inside.
Although the great helm offered vastly superior protection than previous helmets, such as the nasal helm and spangenhelm, it limited the wearer’s peripheral vision, and in addition to being heavy, the mass-produced form (flat-topped without ventilation holes) provided little ventilation and could quickly overheat in hot weather. Knights usually wore the great helm over a mail coif (hood) sometimes in conjunction with a close-fitting iron skull cap known as a cervelliere. The later development of the cervelliere, the bassinet, was also worn beneath the great helm;
Men-at-arms would often remove the great helm after the first clash of lances, for greater vision and freedom of movement in melee combat. The bassinet had a mail curtain attached, a camail or aventail, which superseded the coif. Mail throat and neck defences such as these were made obsolete when plate gorgets were introduced, around 1400.
The bassinet evolved from its early skull cap form to supersede the great helm for combat. The great helm fell into disuse during the 15th century; however it was used commonly in tournaments where a version of the great helm, the frog-mouthed tilting helm, evolved.