The marble pavement of the Cathedral in the Tuscan city of Siena in Italy has been described as one of the marvels of the world. Over the centuries much has been written about its biblical and political characters, the stories depicted in its figurative mosaics, the artists responsible for creating the mosaics, the types of marble used, and the history of their construction. The many frieze patterns framing the figurative mosaics are marvels of geometric design, and yet, they have been conspicuously overlooked in the vast literature concerning this pavement. Here these geometric frieze patterns are analyzed in terms of a typology of patterns of repetition, as well as from the point of view of the optical effects, such as multistable perception, that they engender in the viewer.