The Triumphal Arch at Santa Maria Maggiore
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."
This image in full resolution
Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.