spandrel

An arch springs from two columns and rises into a circle. Around it, a rectangular frame. Between the curve and the rectangle, at each upper corner, a triangular space. That space is the spandrel. It is not a thing anyone designed. It is what's left when an arch is set into a wall.

The spandrel doesn't hold the arch up. Pendentives hold domes; voussoirs hold the arch itself; the spandrel is the leftover. Its shape is determined by the two surfaces it falls between — the arch's outer curve, the rectangle's two inner edges. Change the arch and the spandrel changes. The spandrel has no proportions of its own. It is entirely a negative.

In San Marco in Venice, the spandrels above the arches are mosaicked with the four evangelists. Matthew, Mark, Luke, John. Gold ground. Each figure fitted into the triangle by tilting his halo, bending his shoulder, letting his robe spill into the corner. The geometry is not flattering. A figure built for a panel has to be re-built for a wedge. The mosaicists did it anyway.

In 1979 Stephen Jay Gould and Richard Lewontin wrote a paper using these spandrels as a metaphor. The argument was that evolutionary biology had a tendency to explain every feature of an organism as if it were a designed solution to a selective pressure — when in fact many features exist because they are byproducts of other features. The spandrel is byproduct. The evangelist is what biology mistakes for adaptation.

The biologists' fight isn't the part that holds me. What holds me is that the mosaicists already knew. They did not pretend the spandrel was a designed surface. They did not extend the wall and pretend the arch wasn't there. They took the wedge that the geometry gave them and put an evangelist in it. The frame says this is leftover and the mosaic says I know — here is the gospel.

There is a class of surfaces in architecture that are honest about being leftover. The spandrel. The pendentive's curved triangle. The tympanum above a door, set into the void between a lintel and an arch. The squinch that bridges a square room into an octagon to take a dome. Each one is a shape no one would have chosen, given freely. Each one has, in the great buildings, been given content of unreasonable weight. The transition is the gospel. The leftover is the gospel.

I don't have a moral. I notice the buildings keep doing this. They build their primary structure, and then in the space the primary structure cannot help but produce — the corners, the wedges, the bridges between geometries that don't agree — they put what they most want to say.