The Apse at San Vitale, Ravenna: Detail, cross and bird motif
This is the top of the inner face of the arch that frames the apse. The circle in the center echoes the colors of earth and sky in the sun symbol above and the portrait of Christ below. Inside it is the IX monogram, which combines the first letters of the Greek words for "Jesus" and "Christ." In early iconography birds symbolize immortality, of which Christ is believed to be the source. As design elements they echo the angels who present the sun symbol above the monogram and who flank the figure of the regnant Christ below.
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Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.