The Triumphal Arch at Santa Maria Maggiore

5th century
Basilica of Santa Maria Maggiore, Rome

This was originally the apsidal arch, but in the 13th century the apse was moved back to make space for the transept and this became a "triumphal arch."

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Detail photographs:

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Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.