Parallel lines (two sets for a square or a rectangle) would be the proof. 90 degrees means right angles, right angles means that one line is perpendicular to another line. Meaning ABCD quadrilateral that since each angle forms a 90 degree angle, the lines are perpendicular to each other. It also implies that opposite lines are parallel for both pairs of lines (for instance, you can't have a trapezoid since only one set of lines is parallel for a trapezoid)