(geometry) opposite angle - in the quadrangles, the angle that is not the neighbour to an angle.

mutually; reciprocally; with one another

a quadrilateral (polygon having four sides)

a corner (e.g., of a desk; compare 隅) / an edge / a street corner

crossing, intersection / (genetics) crossover

cross, intersect

(linear algebra) diagonal matrix

(linear algebra) diagonal element; diagonal component

items that come in pairs / sets of furniture etc.

facing, opposed / pair, duo, dyad