spandrel

The space (often more or less triangular) between the outer curve of an arch (the extrados) and a straight-sided figure that bounds it; the space between two contiguous arches and a straight feature above them.

Usual practice then builds a wall above each curved end of the arch, known as a spandrel, which directly joins the bridge's sidewalls. […] With sidewalls and spandrels in place, you can fill the space between them with material up to the height of the bridge deck, and then place the road or railway on top.

Horizontal member between the windows of successive storeys of a tall building.

2002, Kerstin Lang, Seismic Vulnerability of Existing Buildings, vdf Hochschulverlag AG, page 25, Due to the fact that the walls are joined by floors and spandrels, a coupling effect is produced. Depending on the extent of the spandrels, this coupling effect will be bigger or smaller.

The triangular space under a stair; the material that fills the space.

An oriental rug having a pattern of arches; the design in the corners of such a rug, especially in a prayer rug.

A phenotypic characteristic that evolved as a side effect of an adaptation in response to evolutionary pressure.

Another type of theory emphasises the sometimes accidental nature of evolution. A spandrel is a structure arising as a by-product of some other architectural configuration. In evolutionary terms, a spandrel is some trait of an organism which emerges not through direct adaptionist or exaptationist means, but rather as a side effect of some other evolutionary development.