(n) :
the space (often triangular) between the outer curve of an arch (the extrados) and a straight-sided figure that bounds it; the space between two contiguous arches and a straight feature above them; the triangular space under a stair; the material that fills the space
(n) :
the space (often triangular) between the outer curve of an arch (the extrados) and a straight-sided figure that bounds it; the space between two contiguous arches and a straight feature above them; the triangular space under a stair; the material that fills the space