In summary, the two triangular shapes are actually quadrilaterals, and by overlaying the small quadrilateral on top of the large quadrilateral, a fourth quadrilateral is revealed that has two sides equal in length and both parallel to the hypotenuse of the small triangle and the other two are parallel with hypotenuse of the large triangle. This fourth quadrilateral, which has the best one word description of the source of the extra area, is a parallelogram.