spandrel (n.)

"triangular space between the outer curve of an arch and the molding enclosing it," late 15c., apparently a diminutive of Anglo-French spaundre (late 14c.), which is of uncertain origin, perhaps a shortening of Old French espandre "to expand, extend, spread," from Latin expandre "to spread out, unfold, expand," from ex "out" (see ex-) + pandere "to spread, stretch" (from nasalized form of PIE root *pete- "to spread").