In some cases the definition does not explicitly include the fact that it is, firstly, a parallelogram. Instead, the definition might specify that there are four sides and all interior angle are right angles. But, whatever the definition is, from it immediately follows that any rectangle is a parallelogram. read more
parallelogram is a quadrilateral in which both pairs of opposite sides are parallel and congruent. Congruent sides are the same length. A rectangle is a parallelogram with 4 right angles. A parallelogram is not always a rectangle. read more
A parallelogram with all 4 interior angles congruent to each other is called a rectangle. So, straight from a definition we see that any rectangle is a parallelogram with additional property of having all interior angle congruent to each other. read more