The sigma model takes the momenta in the two manifolds T ∗ ∗ Φ Φ and T ∗ ∗ M and solders them together, in that d ϕ ϕ is a solder form. In this sense, the interpretation of the sigma model as an energy functional is not surprising; it is in fact the gluing together of two energy functionals. Caution: the precise definition of a solder form requires it to be an isomorphism; this can only happen if M and Φ Φ have the same real dimension. Furthermore, the conventional definition of a solder form takes Φ Φ to be a Lie group. Both conditions are satisfied in various applications.