Abstract
The general direct coupling between four fermions is studied. It is a linear combination of the five invariants used in β-decay theory. Altering the order of the particles in the Hamiltonian changes only the coefficients of the linear combination. The formalism of charge conjugation is used with the ordinary theory or the Majorana abbreviated theory for neutral particles. This is applied to the study of the decay of the μ-meson into an electron and two neutrinos.