It is worth noting that although the computational stencil depicted in Fig. 1 shows a central point and its eight neighbours on a regular lattice, there is nothing in the present formulation to restrict the method to this type of grid. Indeed all that is required for the WEM are the grid points and information about neighbouring points. In practice, it is often easiest to use the mesh element connectivity generated by (regular or irregular) meshing routines to determine the point neighbours. There may, however, be circumstances when it is more efficient or reliable to determine the neighbours directly from a list of points (or use a combination of surface meshes with distributed points within the domain). In this regard the WEM may be regarded mesh-less or loosely mesh dependent.