spandrel

Imagine the triangular area that appears between the curve of an arch and the frame around it, or the space between two arches. That is the basic idea here. In churches, halls, and old public buildings, this space is often filled with carvings, paintings, or other decoration, turning a necessary part of the structure into something beautiful.

Later, the word moved beyond architecture. It is well known in evolutionary biology as a metaphor for a byproduct: something that appears as a side effect of another change, not because it was directly selected for. So the word can point either to a real built space in a structure or, in a broader sense, to a feature that arose indirectly.

The term "spandrel" comes first from architecture. It usually means the roughly triangular space between the outer curve of an arch and the frame around it, or the space between two neighboring arches. This is not a separate object added afterward; it is a natural result of the way arches meet walls and other supports. Because these spaces are so visible, builders and artists often treated them as places for decoration, adding reliefs, mosaics, paintings, or carved stone.

The idea later became famous outside architecture. In 1979, Stephen Jay Gould and Richard Lewontin used "spandrel" as a metaphor in evolutionary biology. They used it to talk about traits that may exist as byproducts of other evolutionary changes rather than as direct adaptations. In that sense, the word suggests something that came along with a larger design or process, even if it was not the main target of selection.

You will most often see the word in these two settings: architecture and discussions of evolution. In architecture, it names a real space created by arches. In biology and related writing, it names a feature that is better understood as an indirect result than as something shaped for its own purpose. At its core, the word connects necessity with outcome: a space or feature that appears because of how something larger was built.