The cyclic equidiagonal quadrilaterals are exactly the isosceles trapezoids. All of the area formulas for general convex quadrilaterals apply to parallelograms. The orthodiagonal quadrilateral has the biggest area of all convex quadrilaterals with given diagonals. Of all convex quadrilaterals with given diagonals, the orthodiagonal quadrilateral has the largest area. Tangential quadrilaterals are a special case of tangential polygons. Important classes of two-dimensional elements include triangles (simplices) and quadrilaterals (topological squares). Usually the quadrilateral is assumed to be convex, but there are also crossed cyclic quadrilaterals. This is also true for all parallelograms, but not for all quadrilaterals. This formula generalizes Heron's formula for triangles and Brahmagupta's formula for cyclic quadrilaterals. All triangles have a circumcircle, but not all quadrilaterals do. Polygons such as quadrilaterals can be defined by using more than three vertex/texture/normal indices. Simple quadrilaterals are either convex or concave. Some function for quadrilaterals (quads), while others operate on triangles. In Euclidean geometry, the six quadrilaterals illustrated are all different. All triangles can have an incircle, but not all quadrilaterals do.