You probably saw it hundreds of times, in the chaothic traffic of Piazza San Giovanni, just behind the Scala Santa… everyone knows, at least by sight, the Triclinium Lateranensis, or Leoninum, but few know its origin and function. This unique monument is all that survives of a sumptuous banquet hall (the triclinium) built by Pope Leo III (795-816) for the pontifical residence of the Lateran. Nothing remains today of this environment, except the apse; nonetheless, the sources describe it as immense, equipped with “an apse decorated in mosaic (the surviving one) and 10 other smaller painted apses opening on the right and left walls.” The triclinium, paved with precious marbles, housed numerous accubita – that is, the sofas on which diners stretched out to eat, according to Roman usage – and at its center towered a large shell-shaped fountain of red porphyry. The opulent hall was demolished in the 16th century by the will of Pope Sisto V, who decided to preserve only the apse we see today. The absidal mosaic depicts Christ in the act of entrusting the Apostles with their mission; on the left Christ hands the keys to Saint Sylvester and the Labaro to Constantine, while on the right St. Peter gives the stola to Leo III and the insignia to Charlemagne. The square halo of the latter two characters indicates that they were still alive when the work was made.