A glorious, glowing company of saints and angels surround Christ and his mother, watching as he delicately places a golden crown on her head. This is heaven seen through fifteenth-century eyes. Crowns and haloes shimmer with burnished gold; angels with multi-coloured wings swing censers and make music; ranks of saints in gorgeous colours gesture gracefully to each other or gaze adoringly at the culminating drama of the life of the Virgin – her coronation as Queen of Heaven.

This huge polyptych was painted for the high altar of the monastery of San Benedetto fuori della Porta Pinti in Florence between 1407 and 1409. It was originally even bigger and more complex: its main panels and parts of the predella are in the National Gallery, but other parts are scattered in collections around the world.

The panels which make up the main tier – The Coronation of the Virgin, Adoring Saints: Left Main Tier Panel and Adoring Saints: Right Main Tier Panel – were originally constructed and painted as a single panel. We don‘t know exactly when it was split into three, although the altarpiece was probably taken apart when San Benedetto was destroyed in the run up to the siege of Florence in 1529. At some point, two vertical bands about seven centimetres wide were lost between the side and main panels. These missing sections have been reconstructed in the structural elements of the modern frame.

In the predella were scenes from the life of Saint Benedict, patron saint of the monastery: we see him admitting Saints Maurus and Placidus into the Benedictine Order and telling Maurus to save Placidus from drowning. A panel showing Benedict’s death is now on indefinite loan to the National Gallery, and other panels are in the Pinacoteca Vaticana, Rome and the National Museum, Poznań. The pinnacles at the top showed the Annunciation flanking Christ the Redeemer. There were Old Testament prophets in the pilasters at the sides, and a tier showing Abraham, Noah, Moses and David (Metropolitan Museum of Art, New York) between the predella and the main panels.

San Benedetto belonged to the Camaldolites, a reformed branch of the Benedictine Order, famous for their strict lifestyle although they lived among great visual riches. The artist, Lorenzo Monaco, was himself a Camaldolite monk, and this painting is a kind of dress rehearsal for his most important surviving work, the great altarpiece for Santa Maria degli Angeli in Florence. Even this smaller and simpler version measures almost 265 by 360 cm.

Unusually, we know quite a lot about why and when this altarpiece was made. The monastery’s register records how it was commissioned in 1407 by a Florentine citizen, Luca Pieri Rinieri Berri, who was to pay almost the entire cost. In recompense his name was to be included on the altarpiece – a few letters of an inscription can be made out on the grey step of dais – so that he would be remembered in the monks’ prayers. The altarpiece was never to be removed from the high altar, on penalty of a fine of 200 gold florins. It was in place by 1409 when the monks moved choir stalls from a side chapel into the main church.