A Quadrilateral is a polygon having four sides and four angles.
A Parallelogram is a quadrilateral which has its opposite sides parallel.
The side upon which a parallelogram stands and the opposite side are called respectively its lower and upper bases.
A Rectangle is a parallelogram having its angles right angles.
A Square is an equilateral rectangle, [fig. 41].
Fig. 41.
Fig. 42.
