  Current data implies three simple forms of the neutrino mass matrix, each
corresponding to the conservation of a non-standard lepton charge. While models
based on L_e and L_e - L_mu - L_tau are well-known, little attention has been
paid to L_mu - L_tau. A low energy mass matrix conserving L_mu - L_tau implies
quasi-degenerate light neutrinos. Moreover, it is mu-tau symmetric and
therefore (in contrast to L_e and L_e - L_mu - L_tau) automatically predicts
maximal atmospheric neutrino mixing and zero U_{e3}. A see-saw model based on
L_mu - L_tau is investigated and testable predictions for the neutrino mixing
observables are given. Renormalization group running below and in between the
see-saw scales is taken into account in our analysis, both numerically and
analytically.
