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"...the triangular space comprehended between the outer curve or extrados of an arch, a horizontal line drawn through its apex, and a vertical line through its springing; also, the wall-space between the outer moldings of two arches and a horizontal line or string-course above them, or between these outer moldings and the intrados of another arch rising above and inclosing the two. In medieval architecture the spandrels are often ornamented with tracery, sculptured foliage, and the like." —Whitney, 1889 <p>Illustration of a spandrel from the cloisters at Mont Saint-Michel in France.

Sculptured Spandrel from Mont Saint-Michel

"...the triangular space comprehended between the outer curve or extrados of an arch, a horizontal line…