☆? 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.