The smallness and hierarchy in fermion parameters could be the result of selection rules due to an Abelian horizontal symmetry broken by a small parameter. When applied to the lepton sector, then for a large class of models, a number of interesting order of magnitude relations arise: with i < j, m(ν i)/m(ν j) ∼ sin 2 θ ij ; m(ℓ − i)/m(ℓ − j) < ∼ sin θ ij ; m(ν i)/m(ν j) > ∼ m 2 (ℓ − i)/m 2 (ℓ −...