How to prove parallelogram
Two groups of parallelograms whose opposite sides are parallel to each other are parallelograms
Two sets of quadrilaterals with equal opposite sides are parallelograms
A group of parallelograms whose opposite sides are parallel and equal are parallelograms
Two sets of diagonally equal quadrilaterals are parallelograms
A quadrilateral with two diagonals bisecting each other is a parallelogram