A quadrilateral is a polygon with four sides. The quadrilateral ABCD shown in the figure has :

Four sides  AB, BC, CD, and DA

Four Vertices  A, B, C, and D

Four Angles  A (or DAB), B  (or ABC), C (or BCD) and D (or CDA)

Two Diagonals  AC & DB

Parallelogram –

A quadrilateral is called a parallelogram if its opposite sides are parallel. In the figure, ABCD is a parallelogram, in which DC is parallel to AB & DA is parallel to CB. A parallelogram has some special properties which we learn