 A quadrilateral is a flat shape, in one plane, defined by four points at the four corners. A quadrilateral has four sides. The sides can be of any length. And it has four corners. They can be at any angle  to each other so long as they add up to 360 degrees. A rectangle is a special kind of quadrilateral. And then a square is a special kind of rectangle.

To figure out the perimeter of the quadrilateral, you can add the lengths of all the sides together. To figure out the area of a quadrilateral, you have to cut it into shapes that are easier to figure out. You cut it into two pieces running a line from one corner to the opposite corner. Then you have two triangles. You can figure out the area of those triangles. Then add them together to get the area of your quadrilateral.

But unless at least two opposite sides of the quadrilateral are parallel to each other, it’s hard to figure out the height of the triangles. So you won’t really be able to figure out the area of the quadrilateral either. If two of the sides of your quadrilateral are parallel, then you have a parallelogram. That’s easier to figure out the area for.