On the north side of the Duomo there is a small slab of white marble which is inscribed with the Latin words: SATOR AREPO TENET OPERA ROTAS. The five words form a palindrome, which may be read top to bottom, bottom to top, left to right, or right to left.
Unfortunately, the palindrome is not easy to translate, as the meaning of AREPO is not known. The other four words can be translated as: SATOR=sower; TENET=holds; OPERA=work; ROTAS=wheels.
Sator squares appear at sites across Europe. The oldest example was found on a pillar in the ruins of Herculaneum, which was buried in the eruption of Mount Vesuvius in 79 CE. This makes it the world's oldest datable palindrome.
But why should such a square appear on a church?
By rearranging the letters around the central letter N, a Greek cross can be made that reads: Pater Noster (Latin for Our Father, the first two words of the Lord's Prayer) both vertically and horizontally. The remaining letters – two 'A's and two 'O's – can be taken to represent the concept of Alpha and Omega, the beginning and the end, a reference in Christianity to the eternal presence of God.
P A T E R N O S T E R