  Triangular mass matrices for neutrinos and their charged partners contain
full information on neutrino mixing in a most concise form. Although the scheme
is general and model independent, triangular matrices are typical for reducible
but indecomposable representations of graded Lie algebras which, in turn, are
characteristic for the standard model in noncommutative geometry. The mixing
matrix responsible for neutrino oscillations is worked out analytically for two
and three lepton families. The example of two families fixes the mixing angle
to just about what is required by the Mikheyev-Smirnov-Wolfenstein resonance
oscillation of solar neutrinos. In the case of three families we classify all
physically plausible choices for the neutrino mass matrix and derive
interesting bounds on some of the moduli of the mixing matrix.
