In the theory of pions, physics often uses the chiral perturbation theory. Here, the relevant symmetry is the isospin SU(2) symmetry. It is broken by the different masses of u and d quarks as well as by their different charges. The chiral Lagrangian may be extended to an exactly SU(2)-symmetric Lagrangian by promoting these parameters (mass and charge) to fields that break the symmetry spontaneously. Calculations of observables to higher orders may be done with the spurion fields. The final result, at any order of accuracy, is obtained by substituting the right masses and charges.