Does the side in the middle fix the shape of the quadrilateral?
Not stable
Practice makes perfect
By stable, we mean that the figure has a fixed shape. Therefore, if we can change the orientation of the sides, the figure is not stable. A quadrilateral is not stable since there are infinitely many ways you can draw a quadrilateral given four sides. Below we see just two examples.
The extra side in the middle does not fix the shape of the quadrilateral as it can move with the left and right sides of the quadrilateral.
