There are two basic types of restrictions imposed on FDS, the first being approximation and stability restrictions stipulating the convergence of the approximate solution when the mesh size is small enough. The second type of requirement refers to the retention by the FDS of the important properties of the original differential equations. When computing the numerical solution to partial differential equations, difference operators that mimic the crucial properties of the differential operators are usually more accurate than those that do not. Properties such as symmetry, conservation, and duality relationships and identities between gradient, curl, and divergence are all important.