Students can use this applet to discover that opposite sides of parallelograms are congruent, opposite angles are congruent, consecutive angles are supplementary, and that the diagonals bisect each other. They should determine that diagonals in a rectangle are congruent. They should also determine that diagonals in a rhombus are perpendicular and are angle bisectors. Naturally all properties of rectangles and rhombuses apply to squares.
