On the wall of a house in Calle del Forno is walled a terracotta bas-relief with five words arranged in a square of twenty-five cells: SATOR, AREPO, TENET, OPERA, ROTAS. It reads in every direction: left to right and right to left, top to bottom and bottom to top, and in the centre the word TENET forms a cross. It is the Sator square, the most famous palindrome in history.