Pope Leo III (795-816) wanted to create two Triclinia modelled on the rooms of the imperial palace of Constantinople, decorated in mosaics and lined with marble, in which he could create a space for the reception of large official banquets. The largest triclinium, measuring some 68 metres in length, was located perpendicular to the north side of the basilica and was intended for official banquets. It was richly decorated with marble and semicircular niches in which the accubita, period sofas and tables were arranged. The whole was enriched by a large fountain made of red porphyry.
In the sixteenth century, the large hall of the triclinium was demolished, and only the large apsidal mosaic was conserved, which was restored in 1625 at the behest of Cardinal Francesco Barberini. However, this mosaic was dismantled and placed in the church storerooms due to the construction of the façade of the Basilica in 1700. It was not until 1743 that Pope Benedict XIV commissioned Ferdinando Fuga to design an aedicule that could once again house it, which is why the papal coat of arms of this Pontiff stands above it.
The mosaic depicts various scenes from the life of Christ and the Church. Indeed, at the centre we see Jesus who entrusts the Apostles with their mission, also confirmed by the underlying Gospel reference with the final words of Chapter 28 of the Gospel of Matthew. In the arch, on the left, Christ on the throne consigns the keys to Pope Sylvester and the imperial labarum to Constantine, whereas on the right, there is the figure of Saint Peter consigning the pallium to Pope Leo III and the imperial insignia to the first sovereign of the Holy Roman Emperor, Charlemagne. Indeed, it was Pope Leo III who proclaimed Charlemagne emperor.
The inscription that runs around the perimeter of the apse bears the words Gloria in Excelsis Deo.
