What defines a parallelogram?

A parallelogram is defined as a quadrilateral in which both pairs of opposite sides are parallel. This characteristic is essential because it ensures that the opposite sides are not only parallel but also equal in length. In a parallelogram, the properties of parallelism lead to specific angles, where opposite angles are equal, and adjacent angles are supplementary.

Understanding this definition helps in distinguishing parallelograms from other quadrilaterals. For instance, the interpretation of having all sides of equal length pertains more specifically to squares and rhombuses, not to parallelograms in general. Similarly, while having at least one right angle is a property of rectangles—a specific type of parallelogram—it does not define all parallelograms. Therefore, the defining property of having exactly two pairs of opposite parallel sides is both broad enough to encompass all types of parallelograms and specific enough to distinguish them from other quadrilaterals.