The general theory and formulation of Part 1 can be applied to a whole family of element shapes. However, quadrilaterals and triangles which are compatible with each other are found to be most useful in idealising general structures. The lowest order polynomial shapes of this kind which are generally curved are the ‘biquadratic’ quadrilateral and the ‘quadratic’ triangle. These have nodes (or nodal pairs for the sandwich elements) situated at the corners and the mid-sides of the ‘parent’ elements. The quadrilateral therefore has eight nodal pairs.