At the far west end of the basilica is the tribune, which centers on the Cathedra of St. Peter. The enormous gilded bronze monument was designed by Bernini in 1666 to enclose an oak throne donated by Carolingian ruler Charles the Bald upon his coronation in St. Peter's in 875. The legs of the throne are decorated with finely pierced ivory bands made in the School at Tours. The 18 ivory plaques on the front of the chair were added slightly later, and show the 12 Labors of Hercules and six monsters. Bernini's monument is topped by a yellow window featuring the Holy Spirit as a dove surrounded by 12 rays, symbolising the apostles. To the right of the chair are St Ambrose and St Augustine (fathers of the Latin church), and to the left are St Athanasius and St John Chrysostom (fathers of the Greek church).