A diaphragm arch is a transverse wall-bearing arch forming a partial wall dividing a vault or a ceiling into compartmentsTechnical Terms and the Understanding of English Medieval Architecture" E. C. Fernie, Architectural History Vol. 44, (2001), pp. 13-21 while also bracing the walls.
When used under a wooden roof (with solid spandrels) it has the advantage of providing a partial firebreak. Architecture in Britain: the Middle Ages, Volume 12, p. 210 It was first used in Roman Syria, during the 2nd century AD.Arte medievale, 1987 The diaphragm arch is present in Islamic, Carolingian, Ottonian, Romanesque and Gothic architecture (a solution often employed in Catalan Gothic, for instance).
See also Islamic architecture Romanesque architecture Kucheh
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