The Apse Mosaic at Sant'Apollinare in Classe: Detail, the Angel Gabriel
There is a corresponding image of St. Michael on the left frame of the apse. Each angel holds a military standard called a labarum, as they usually do when they flank a sacred space or personage. The most traditional labara have a chi-rho symbol at the top and something related to Constantine's adoption of Christianity on the cloth. Here, however, there is no chi-rho, and to mark the sacred space beneath the apse the banner on the labarum bears the Greek words ΑΓΙΟC ΑΓΙΟC ΑΓΙΟC, "Holy Holy Holy." That phrase appears in Isaiah 6:3, Revelation 4:8, and the Sanctus, a prayer in the Roman Catholic Liturgy.
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Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.