spandrel
The triangular-shaped, sometimes ornamented area between the extrados of two adjoining arches, or between the left or right extrados of an arch and the rectangular framework surrounding it. Also, a panellike area in a multistory frame building, between the sill of a window on one level and the head of a window immediately below.
First 10 image search results of spandrel architecture
Jargon of the day: negative/positive space
See: Solid/Void
