spandrel

The word spandrel (also spelled spandril) likely derives from Anglo-Norman French spaundre or espaundre, related to Old French espandre, meaning 'to spread out' or 'to expand,' ultimately from Latin expandere. The spandrel is the roughly triangular area between the outer curve of an arch, the horizontal line drawn from its apex, and the vertical line rising from its springing point. Alternatively, it describes the space between two adjacent arches and the horizontal molding above them. It is, in essence, leftover space -- the geometric consequence of fitting a curved form into a rectangular frame. Yet this leftover has proven to be one of architecture's most important surfaces, a canvas that builders from ancient Rome to the present have filled with some of their most extraordinary decorative work.

Roman architects were among the first to exploit spandrels systematically. On triumphal arches like the Arch of Constantine, the spandrels flanking the main archway were filled with relief sculptures of winged victories, river gods, or seasonal figures. The spandrel became a designated location for narrative art, a space where the structural logic of the arch created a natural frame for imagery. Byzantine architects continued and expanded this tradition: the pendentive spandrels of Hagia Sophia, which transition from the square base to the circular dome above, are among the most celebrated architectural surfaces in history, covered in golden mosaics that seem to hover in space. In Islamic architecture, arch spandrels became fields for some of the most intricate geometric and arabesque decoration ever produced.

Gothic and Renaissance European architecture continued to treat the spandrel as a prime decorative surface. The spandrels of the Doge's Palace in Venice display carved figures within quatrefoil frames. English Perpendicular Gothic churches filled their arch spandrels with carved angels, heraldic devices, and tracery. The spandrel offered a defined, contained space that demanded filling -- its triangular geometry creating natural compositional challenges that inspired inventive solutions. Even when the rest of a facade was relatively plain, the spandrels often received concentrated ornamental attention, as if the space between arches demanded more decoration precisely because it was secondary.

In 1979, evolutionary biologists Stephen Jay Gould and Richard Lewontin borrowed the word spandrel for one of the most influential metaphors in modern science. They argued that many features of organisms, like the spandrels of San Marco in Venice, are not direct products of natural selection but rather byproducts of other adaptations -- necessary consequences of structural constraints rather than designed features. The architectural term became a biological concept, describing traits that fill available space without having been specifically selected for. The word's journey from building to biology captures something essential about spandrels themselves: they are the spaces that emerge when primary forms create secondary geometries, the triangles that appear whenever a curve meets a corner.