☆? Two groups of quadrangles with parallel opposite sides on the same plane are called parallelograms.

☆? Two groups of quadrangles with equal opposite sides on the same plane are called parallelograms.

☆? A group of quadrilaterals with parallel and equal opposite sides on the same plane is called parallelogram.

☆? A group of quadrangles with equal diagonal angles on the same plane is called parallelogram.

Square and rectangle are special parallelograms.