Abstract. In this paper we generalise results regarding the order of accuracy of finite difference operators on Summation-By-Parts (SBP) form, previously known to hold on uniform grids, to grids with arbitrary point distributions near domain boundaries. We give a definite proof that the order of accuracy in the interior of a diagonal norm based SBP operator must be at least twice that of the boundary stencil, irrespective of the grid point distribution near the boundary. Additionally, we prove that if the order of accuracy in the interior is precisely twice that of the boundary, then the diagonal norm defines a quadrature rule of the same order as the interior stencil. Again, this result is independent of the grid point distribution near the domain boundaries.Key words. Finite difference schemes, summation-by-parts operators, numerical differentiation, quadrature rules, order of accuracy AMS subject classifications. 65M06, 65N061. Introduction. Summation-By-Parts (SBP) operators, applied in the discretisation of systems of partial differential equations have received considerable attention since they lead to provable energy stability [25], and recently, entropy stability [6] for well-posed problems. Finite difference stencils on SBP form were first introduced in [14,15] based on central difference schemes of order 2 and 4. Later, operators with minimal bandwidth using stencils of order 6 and 8 were developed in [22]. In [3,19] SBP operators of orders up to 8 for both first and second derivatives were presented.The SBP concept has been extended to methods outside the finite difference community. These include spectral collocation and spectral element methods [2,8,9] as well as correction procedures via reconstruction [21]. Further, the finite difference class of SBP operators has been enlarged to multidimensional operators similar to Galerkin methods [12] as well as to grid dependent stencils akin to element based methods [5]. In this paper we restrict our attention to fixed stencil finite difference schemes and do not consider these extended approaches further.Implicit to the definition of an SBP operator is the notion of a discrete norm. If this norm is represented by a diagonal matrix, the associated operator is referred to as a diagonal norm based SBP operator. To avoid stability issues on curvilinear grids [23], and in general for problems with variable coefficients [20], finite difference operators on SBP form are in practice usually based on a diagonal norm.The focus in this paper is on SBP operators consisting of a repeated central difference stencil in the interior, and one-sided stencils near boundaries and interfaces. We introduce the notation SBP(τ ,2s) to refer to such an operator that is of order τ near the boundary and of order 2s in the interior. For completely uniform grid distributions, the accuracy of SBP(τ ,2s) is known to be dictated by two main constraints; we will formalise these as Theorems 4 and 5 in the next section. The first one states that s ≥ τ , i.e. the order of accuracy of the int...