 A convex quadrilateral is

De Villiers defines a rectangle more generally as any quadrilateral with axes of symmetry through each pair of opposite sides.This definition includes both right-angled rectangles and crossed rectangles.Each has an axis of symmetry parallel to and equidistant from a pair of opposite sides, and another which is the perpendicular bisector of those sides, but, in the case of the crossed rectangle, the first axis is not an axis of symmetry for either side that it bisects.