Entrance from the Museo e Tesoro del Duomo, on the left of the bell tower
The Iron Crown has been preserved from the Middle Ages to the present day; it is composed of six plates of gold decorated with rosettes in relief, ouches of gems and enamels fixed on a circle of metal, from which it takes its name of "iron". An ancient tradition, already reported by sant 'Ambrogio at the end of the IV century, identifies this metal ring with one of the nails used in the crucifixion of Christ: a relic, then, that St. Helen would have found in 326 during a trip to Palestine and inserted in the diadem of her son, the Emperor Constantine. The tradition, which binds the Crown to the passion of Christ and the first Christian emperor, explains the symbolic value attributed by the kings of Italy (or the would-be, as the Visconti), which would use it in coronations certifying the origin from God of their power and their link with the Roman emperors. Recent scientific investigations do envisage that the Crown is derived from goldsmith carried out between the IV-V and IX centuries and it could have be a real sign in late antiquity, perhaps by the Ostrogoths, passed to the Lombard kings and finally to the Carolingian, who would have it restored and donated to the Cathedral of Monza. Since then the history of the tiara was inextricably linked to that of the cathedral and the city. In 1354, for example, Pope Innocent VI sanctioned as undisputed right of the Cathedral of Monza to host the coronation of the kings of Italy, and in 1576 St. Charles Borromeo instituted the worship of the Holy Nail, so is to make the official recognition of the tiara as a relic and to tie it to another Holy Nail, preserved in the cathedral of Milan, which the ancient tradition tells sant'Elena would have forge shaped a bite for the horse of Constantine, as further metaphor of divine inspiration in command of the Empire.