The nature of parallelogram: the opposite sides of parallelogram are parallel and equal, the diagonals of parallelogram with complementary adjacent angles are equal, and the sum of squares of the diagonals of parallelogram is equal to the sum of squares of four sides. A parallelogram is a figure with central symmetry, and the center of symmetry is the intersection of two diagonals. The internal and external angles of parallelogram and equilateral parallelogram include

Rectangular rhombus

Square and general parallelogram General parallelogram has no symmetry axis.