  We propose a parametrization of the lepton mixing matrix in terms of an
expansion in powers of the deviations of the reactor, solar and atmospheric
mixing angles from their tri-bimaximal values. We show that unitarity triangles
and neutrino oscillation formulae have a very compact form when expressed in
this parametrization, resulting in considerable simplifications when dealing
with neutrino phenomenology. The parametrization, which is completely general,
should help to establish possible relations between the deviations of the
reactor, solar and atmospheric mixing angles from their tri-bimaximal values,
and hence enable models which predict such relations to be more directly
compared to experiment.
