The triangular or curved space between structural elements, such as between the curve of an arch and the rectangular frame surrounding it, or the panel area between windows on different floors of a building.
From Anglo-French 'espaundre' (to expand), entering English in the 15th century. Originally described the space between arch curves and their rectangular enclosures in medieval architecture.
Louis Sullivan turned spandrels into art in his Chicago skyscrapers - those decorative panels between office floors became canvases for intricate ornamental designs that celebrated the vertical nature of tall buildings. Even today's glass towers have spandrel panels, though they're usually just opaque glass hiding mechanical equipment.
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