Consider a compact connected symplectic manifold $(X,\omega)$ of dimension $2n$ together with a Hamiltonian action of the torus $T^{n}$. Then one has an integrable system $\pi:X\to B_{0}$, where $\pi$ is the moment map of the action and $B_{0}=(Lie(T^{n}))^{\ast}\simeq{{\bf R}}^{n}$. Furthermore, it is well-known that $B=\pi(X)$ is a convex polytope and $B^{sm}$ is the interior of $B$.