Unlike most duomi, it is not in fact a cathedral, as Monza has always been part of the Diocese of Milan, but is in the charge of an archpriest who has the right to certain episcopal vestments including the mitre and the ring.

[2] According to the legend she had made a vow to build a church dedicated to Saint John the Baptist, and when riding along the banks of the Lambro River, she was halted by a dove who told her Modo (Latin for "now"), to which she replied Etiam ("yes").

In the late 14th century, the side chapels were added and, as designed by Matteo da Campione, the Pisan-Gothic style west front in white and green marble was begun.

In the upper section is portrayed Theodelinda offering to John the Baptist the Iron Crown of Lombardy, together with her kneeling husband Agilulf and their children Adaloald and Gundeberga.

Placed in the old case in Cornu Epistolæ is the opus 617 from the prestigious Swiss organ manufacturer Metzler Orgelbau [4] with fully mechanical transmission, with 29 stops distributed in the two manuals and pedal.

The decoration of the presbytery and choir is the greatest pictorial achievement of the seventeenth century and sees Stefano Danedi known as Montalto, Isidoro Bianchi, Carlo Cane and Ercole Procaccini the Younger at work, with quadrature by Francesco Villa.

The ten paintings in the central nave with Stories of Theodolinda and the Iron Crown, made between the 17th and 18th centuries, belong to various painters, including Sebastiano Ricci, Filippo Abbiati and Andrea Porta.

The final episode consists of the intervention in the cathedral by Carlo Innocenzo Carloni, the great master of international rococo, already active in Austria, Germany and Bohemia.

Between 1738 and 1740, according to a program established by the Jesuit Bernardino Capriate, he decorated the vaults of the aisles, the triumphal arch and the western walls of the transept.

Their 18th-century style uniform, of blue wool with gold braiding and a belt buckle with an image of the Iron Crown, is unchanged from that approved in the edict, except that since the Napoleonic period the bicorne hat has replaced the earlier tricorne.