spandrel

n. [ From Span. ] 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. [ 1913 Webster ]

2. A narrow mat or passe partout for a picture. [ Cant ] [ 1913 Webster ]