SPANDREL

spandrel. Spandrel, n. the space between the curve of an arch and the right lines inclosing it

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Span″drel (?), n. [[From Span.]] 1. 1. (Arch.) The irregular triangular space between the curve of an arch and the inclosing right angle; or the space between the outer moldings of two contiguous arches and a horizontal line above them, or another arch above and inclosing them.
2. 2. A narrow mat or passe partout for a picture.