The midpoints of the sides of any quadrilateral with perpendicular diagonals form a rectangle. Therefore, every convex kite is a tangential quadrilateral. Hence that point is the Fermat point of a convex quadrilateral. They all have triangular and quadrilateral faces. The intersection of the bimedians is the centroid of the vertices of the quadrilateral. These are also the diagonals of the "contact quadrilateral". A non-planar quadrilateral is called a skew quadrilateral. A Lambert quadrilateral in the hyperbolic plane has 3 right angles. An example of a concave quadrilateral is the dart. The figure shows examples of the various cases for a planar quadrilateral linkage. The triangle, quadrilateral and nonagon are exceptions. These are the triangle and the quadrilateral. These points can be used to define an Euler line of a quadrilateral. For an asymmetric quadrilateral this tiling belongs to wallpaper group p2. Consider the figure of a quadrilateral whose coordinates are labeled in counterclockwise order.