Could some of you give me a short explanation, what the index sets in MatCreateNest can be used for? I already had a look to the examples and tests, but all of them just put PETSC_NULL for the index sets or create the standard contiguous index sets by hand (ksp/ksp/example/tests/ex11.c). Does the index sets influence the row/column indices for the final MatNest? The reason why I'm asking for is that I want to have a special order in my MatNest. My scenario is as follows: I have a 2x2 MatNest. The first block corresponds to all interior nodes on all subdomains. The second block corresponds to the boundary nodes between all subdomains. This works fine, also with the fieldsplit preconditioner! Now I would like to permute the global index of the second block in that way, that all edge boundary nodes are listed first and finally all vertex boundary nodes (its a 2D problem). So each rank local matrix of the second block is no more contiguous. I'm not sure, if I need to change the creation of the second block matrix or the creation of the nested matrix. I thought may be it is possible to do it with the index sets when creating the MatNest? Thomas