The Apse at Monreale Cathedral: Detail, The Prepared Throne
The symbol is at the very top of the vault above the main altar, above the Christ Pantocrator. The Greek inscription is H ET[O]HMACÍA, "The Preparation," the name for the "Prepared Throne" symbol in Eastern churches. Unlike most such symbols, this one lacks the usual book on the throne to represent the Son. The Father is represented by the empty throne, the Holy Spirit by the dove. The symbols for the Son are references to his Passion: behind the dove is a cross with a garland that represents the crown of thorns; on either side of the cross are a spear and a pole with a sponge; and in front is a pot for the vinegar given to Christ, here shown with four nails.
Inside the outermost of the dove's circles is the inscription REST. 1979, marking the mosaic's restoration in 1979.
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Photographed at the cathedral by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.