Hi, PetscErrorCode PETSCDM_DLLEXPORT AOCreateBasic(MPI_Comm comm,PetscInt napp,const PetscInt myapp[],const PetscInt mypetsc[],AO *aoout) When mypetsc is PETSC_NULL, it uses natural ordering. Does the natural ordering mean each processor has a contiguous set of indices and process with rank 0 begins with indice 0? Yixun