Theory:

A parallelogram whose each angle is a right angle is called a rectangle.
Now we are going to define a rectangle \(ABCD\) as a quadrilateral is a parallelogram whose each interior angle is \(90°\).
 
Theory_4_1.png
 
Thus, if \(ABCD\) is a rectangle then \(AB=BC=CD=AD\) and \(AB||CD\) and \(BC||AD\).
Important!
A rectangle has all the properties of a parallelogram with interior.
In a rectangle, the following properties are true:
  1. The opposite sides are parallel and equal in length.
  2. The interior angles of the rectangle is \(90°\).
  3. The diagonals are equal in measure and bisect each other.
  4. The adjacent angles are supplementary.