Why parallelograms are quadrilaterals but quadrilaterals are sometimes parallelograms?

Parallelograms are a specific type of quadrilateral characterized by having opposite sides that are both parallel and equal in length. In contrast, quadrilaterals are any four-sided polygons, which can include a variety of shapes such as rectangles, trapezoids, and rhombuses. While all parallelograms fit within the broader category of quadrilaterals, not all quadrilaterals meet the criteria to be classified as parallelograms. Thus, while the former is a subset of the latter, the reverse is not true.