The diagonal divides a parallelogram into two congruent triangles. In a parallelogram opposite sides are equal, and conversely. In a parallelogram opposite angles are equal, and conversely. A quadrilateral is a parallelogram if a pair of its opposite sides is parallel and equal. In a parallelogram, the diagonals bisect each other and conversely. In a triangle, the line segment joining the mid points of any two sides is parallel to the third side and in half of it and (motivate) its converse.
