The mosaic that adorns the facade of Santa Maria in Trastevere probably dates back to the 12th century. Its meaning has long been a puzzle.
In the centre of the mosaic sits the Virgin Mary suckling the Christ Child, a type of image known as the Madonna Lactans. At the base of her throne kneel two tiny figures, whose tonsured heads indicate an ecclesiastical background.
Mary is flanked by ten female figures. Eight of the ten are crowned and clad in royal regalia, while the two to the Virgin's left do not wear crowns and are more simply dressed. All ten figures have halos and are carrying lamps, but the lamps of the two uncrowned figures are unlit! The iconography of this scene is unique and its significance unclear. The mosaic was once thought to illustrate the story of the Wise and Foolish Virgins, but this is now disputed.
Most of the mosaics in the apse of the church also date back to the 12th century, specifically to the reign of Pope Innocent II (r. 1130-43).
In the conch of the apse we see, sitting side by side on the same throne, the Virgin Mary and Christ. The Hand of God (Dexter Dei) emerges from a wreath above Christ's head. On the left stand Pope Innocent II (holding a model of the church), St Lawrence and Pope St Callixtus, while on the right stand St Peter, Pope St Cornelius, Pope St Julius and St Calepodius.
On the choir arch we see, in addition to the emblems of the four Evangelists, two Old Testament prophets, Isaiah and Jeremiah. Each prophet holds a scroll. Isaiah's reads: ECCE VIRGO CONCIPIET ET PARIET FILIVM(Behold, a virgin will conceive and give birth to a Son) and that of Jeremiah: CHRISTVS DOMINVS CAPTVS EST IN PECCATIS NOSTRIS (Christ the Lord is caught in our sins). Above each prophet is a small caged bird, a curious image the meaning of which is a puzzle. Does the answer lie in the words on Jeremiah's scroll? At the top of the arch there is a cross, with the Greek letters A and Ω (Alpha and Omega), which is flanked by the Seven Candlesticks of the Book of Revelation.
At a lower level are six mosaics (c. 1291) depicting scenes from the life of the Virgin Mary, the work of the Roman painter and mosaicist, Pietro Cavallini (c.1250-c.1330). From left to right there is the Birth of Mary, the Annunciation, the Nativity, the Adoration of the Magi, the Presentation in the Temple and the Dormition.
The mosaics were commissioned by Cardinal Bertoldo Stefaneschi, who is depicted in the presence of the Virgin and Child, St Peter and St Paul.