A quadrilateral is a closed plane figure or three-dimensional figure surrounded by four line segments that are not on the same line. It is called a quadrilateral and consists of a convex quadrilateral and a concave quadrilateral. The quadrilateral obtained by connecting the midpoints of any quadrilateral in turn is called a midpoint quadrilateral, and the midpoint quadrilateral is a parallelogram. The midpoint quadrangle of a diamond is a rectangle, the midpoint quadrangle of a rectangle is a diamond, the midpoint quadrangle of an isosceles trapezoid is a diamond, and the midpoint quadrangle of a square is a square.

The four vertices of the concave quadrilateral are on the same plane, the opposite sides do not intersect and are in a straight line with one side, and some of the other sides are on different sides.

The quadrilateral obtained by connecting the midpoints of the sides of the quadrilateral in turn is called the midpoint quadrilateral. No matter how the shape of the original quadrangle changes, the shape of the midpoint quadrangle is always a parallelogram. The shape of the midpoint quadrangle depends on the diagonal of the original quadrangle. If the diagonal of the original quadrangle is vertical, the midpoint quadrangle is rectangular; If the diagonals of the original quadrangle are equal, the midpoint quadrangle is a diamond; If the diagonals of the original quadrilateral are all vertical and equal, the midpoint quadrilateral is a square.