A parallelogram is a planar figure bounded by four straight lines (a quadrilateral) in which both pairs of opposite sides are parallel.
It can be proved that in any parallelogram opposite sides and opposite angles are equal. Each diagonal divides the parallelogram into two congruent triangles.
In the figure line segment AB is parallel to DC and AD is parallel to BC. Further AB is equal to DC (segments have same length) and AD is equal to BC. The angle ∠DAB is equal to ∠BCD and ∠ADC is equal to ∠CBA. The triangle ABD is congruent to CDB (two sides and included angle are respectively equal). Similarly, DAC is congruent to BCA.