For many problems that involve the discrete representation of a function on a two-dimensional region, a single rectangular grid is inappropriate. However, any compact region with a smooth boundary can be discretised using a composite grid, which provides boundary-fitted coordinates and allows stretched coordinates for boundary layers and internal layers wherever these are needed. We have developed interactive software for specifying composite grids, and general-purpose subroutines for using them in discretising PDE boundary-value problems. The discretisation of an elliptic BVP with an internal layer and of a supersonic flow problem are given as examples of the uses of composite grids and the capabilities of the software.
