Roughly triangular area on either side of an arch , bounded by a line running horizontally through its apex, a line rising vertically from the springing of the arch, and the exterior curve of the arch.
When arches adjoin, the entire area between their crowns and springing line is a spandrel. If filled in, as is ordinarily the case, the result is a spandrel wall; in medieval architecture this was usually ornamented. In buildings of more than one story, the spandrel is the area between the sill of a window and the head of the window below it. In steel or reinforced-concrete structures, a deep spandrel beam may span across this area. The triangular area of space beneath a stair is also known as a spandrel.