A quadrilateral with congruent diagonals is a rectangle or a square. All four angles of a rectangle and a square are right angles, making the diagonals congruent. The diagonals of a rectangle and a square also bisect each other.
Rectangles and squares are parallelograms with both pairs of opposite sides parallel and equal. The area of a rectangle is given by A=length x width and the perimeter is given by P=2(length+ width).
A square has four equal sides. The area of a square is given by A=side x side, or side squared. The perimeter of a square is given by P= 4 x length of the side of the square.
The opposite angles of a rectangle are also congruent. Any two consecutive angles of a rectangle and a square are supplementary: their measures add up to 180 degrees. Every square is a rectangle but not every rectangle is a square.