Elements of plate bending and membrane stretching. (a) Element of plate bending with quadratic deflection. Conformity requires 2 nodal displacements on each edge. (b) Dual element for membrane stretching with quadratic Airy's function. Diffusivity requires nine nodal stress function connectors. (c) Corresponding element for membrane stretching constant stress field. Boundary equilibrium (diffusivity) requires six nodal forces.