Carlo Crivelli, 'The Annunciation, with Saint Emidius', 1486

The Archangel Gabriel descends from God to tell the Virgin Mary that she is to bear a child – a moment known as the Annunciation – but has been distracted from his mission by a bishop saint, who has stopped him in the street of a Renaissance town. This is Saint Emidius, patron saint of Ascoli Piceno in the Italian Marches.

Ascoli was ruled by the pope but was granted a degree of self government in 1482. The news arrived on 25 March, the feast of the Annunciation. From then on, the feast was celebrated with a great procession from the town up to the convent of the Observant Friars, for whom this altarpiece was made. The coats of arms along the base of the painting are those of the town, the pope and the bishop, while the inscription, ‘LIBERTAS ECCLESIASTICA’, was the title of the papal bull granting Ascoli its rights.

This painting is unique in showing a local saint effectively intervening in a biblical event. Emidius balances a model of Ascoli, whose towers are still recognisable today, on his knee. Just the other side of the wall we see Mary kneeling in a splendid Renaissance palace, about to be struck by a heavenly bolt of light which will transform her into the mother of God.

Around them the town goes on, oblivious. Only a small child peering round the corner of the steps and a young man gazing up at the sky have noticed the flash which carries the dove of the Holy Ghost down from heaven, through a convenient golden hole in the wall and to Mary. The wind of its passing has disturbed the doves, who flutter from their perches, and ruffled the oriental carpet hanging over the wall above Mary’s chamber. A man on the bridge reads a message handed to him by another man, perhaps the news of the papal bull. It has been delivered by the carrier pigeon in the cage beside them – a witty analogy of the Annunciation itself.

This is Crivelli’s best-known painting and a masterpiece of decoration and perspective. He’s combined classical architecture and Renaissance ornament with an interest in the depiction of everyday objects, similar to Netherlandish painting. Most of Crivelli’s altarpieces were polyptychs with gold backgrounds, but this is a pala, an altarpiece with a single, unified surface. This format allowed Crivelli to show both his mastery of linear perspective and his gift for descriptive detail.

The vanishing point is, unusually, located to the left rather than in the centre of the composition, so that Crivelli can put a close-up of the inside of Mary’s house next to a view down the narrow street beside it. Like many artists, Crivelli used ideas from other painters. Italian artists such as Mantegna were experimenting with radical perspective from the 1450s, and Crivelli probably took the idea for the composition of Gabriel in the street and the Virgin in her house from Giovanni Angelo d'Antonio’s Annunciation, made for the main church in Camerino in about 1455. But he’s transformed his model completely, so that the angel, the saint and the Virgin appear almost as if they were ordinary people going about their daily business. Objects and people cast shadows – the peacock, the fluttering carpets, even Emidius himself – adding to our sense of looking into a real space. In the background people chat, a maid balances a pot on her head and spins wool on a distaff, while mortar drips down a recently repaired wall.

Mary’s room is full of beautifully observed household objects which give the painting a non-religious feel. The books piled on the shelf above her bed are rotated at different angles to show off Crivelli’s trademark skill at foreshortening, while the candle and vase cast distinct shadows on the wooden panelling. Mary kneels at a prie-dieu (a prayer desk) and reads from a book whose pages flutter in the breeze – she was thought to have been reading Isaiah’s prophecy of the birth of the Messiah at the moment of Gabriel’s arrival.

Mary herself is a fashionable fifteenth-century lady. She wears a tight bodice embroidered with gold flowers and foliage, and her linen underdress emerges in puffs from her slashed sleeves. Her head is bare, aside from a jewelled coronet, a reminder that she is both a virgin and Queen of Heaven: only unmarried girls and royalty had uncovered hair. Many of the details dotted around the painting had symbolic meaning at the time. The goldfinch in its cage and the peacock perched were symbols respectively of Christ’s Passion and Resurrection, while the lily Gabriel holds refers to Mary’s chastity and spiritual purity. Even the glass vase was a metaphor for the mystery of Christ’s conception, as light passes through glass without fracturing it.

As well as commemorating their political and financial autonomy, the altarpiece was also an appeal for divine aid. The model city held by Emidius recalls miniature cities offered by communes in the Marche at the important shrine of the Virgin at Loreto, on the Adriatic coast north of Ascoli. From 1484 Ascoli was devastated by a plague so severe that the Annunciation Day procession had to be cancelled and many of the citizens fled. It also plunged into a war with its long-standing rival, the city of Fermo.

We see Crivelli’s signature and the date on the pilasters by the doorway into the Virgin’s house, while fruit – his trademark – balances precariously on the marble shelf at the bottom. Crivelli loved playing games with his viewers, suggesting that his paintings were both flat and three-dimensional. The fruit here casts shadows both on the shelf and on the surface of the painting, as do the fruit and flowers in The Vision of the Blessed Gabriele.