A quadrilateral is a 4-sided polygon.  Windows and doors are special quadrilaterals called rectangles.  Carpenters have a method of making sure the frames of windows and doors are indeed rectangles (have right angles).  If the sides are the same length and the top and bottom are the same, they measure the diagonals to see it they are also the same.  This method works very well, and is based on the following theorem:

THEOREM:     If opposite sides of a quadrilateral are congruent and the diagonals are congruent, then that quadrilateral is a rectangle.

To prove it, suppose , , and diagonal  is congruent to diagonal  in quadrilateral ABCD.  Then by SSS, triangles ADC and CBA are congruent, so  and .  Since  and  are alternate interior angles for  and  with transversal , and  and  are alternate interior angles for  and  with transversal , it follows that the opposite sides of the quadrilateral are parallel (thus it is a parallelogram).  Also by SSS, , so .  But these are same-side interior angles for parallel lines  and , and therefore add to 180^o.  Since they are equal, each must be 90^o.  By the same reasoning, we conclude that the other two angles of the quadrilateral are right angles.