In 2009, 42 fragments of white marble with Cosmatesque decorative inserts were found in the store rooms of the Vatican Museums. Inscriptions on some of these fragments identified their source as the medieval portico of San Giovanni in Laterano, destroyed in 1732. Shortly after the dismantling of the ancient façade, the slabs were reused in the new floor of the Basilica's Portico Sistino. Research conducted on these fragments demonstrated that they were what remained of the frieze on the front of the portico, consisting of an alternation of panels and disks delimited by a band in Cosmatesque work and filled with mosaics, some of which are figurative and with short inscriptions. The fragments were reassembled where possible and restored by the Restoration Laboratories of Marble and Mosaics of the Vatican Museums, while investigations were carried out on the compositional materials from the Diagnostic Laboratory of the Vatican Museums. At the same time, bibliographical and archival research has been carried out which has allowed for a visualisation of the whole scheme.

In the collective memory of Western Christendom it is the basilica of St Peterߣs that is the mother church of Latin Christianity. However, this rank officially appertains to the Basilia of St John Lateran built by Constantine as the Cathedral of Rome. The fact that today the Lateran is no longer perceived as the Cathedral of Rome might go back to the 14th century, when the Popes returned from Avignon and re-established their Roman residence near the basilica of St Peter. But already as early as the sixth century Pope Symmachus (498-514) erected episcopia on both sides of the atrium of St. Peterߣs and copied the display of the Lateran baptistery with its three oratories in the baptistery of St. Peter. This was a highly symbolic act, directed against his opponent Laurentius, who was elected antipope in the very same year as Symmachus. In later centuries it was mostly in periods of conflict that the two basilicas – St John Lateran and St Peterߣs – assumed imporant roles as places of display of rival interests. This chapter investigates the visual strategies of these rival claims through its study of architecture, tombs, relics and images.