  The recent experimental measurements of the solar neutrino mixing angle
$\theta_{sol}$ and the Cabibbo mixing angle $\theta_C$ reveal a surprising
relation, $ \theta_{sol}+\theta_C \simeq \frac{\pi}{4} $. We review that while
this empirical relation has been interpreted as a support of the idea of grand
unification, it may be merely accidental in the sense that reproducing the
relation at a low energy in the framework of grand unification may depend
strongly on the renormalization effects whose size can vary with the choice of
parameter space. We note that the lepton mixing matrix derived from
quark-lepton unification can lead to a shift of the complementarity relation at
low energy. While the renormalization group effects generally lead to additive
contribution on top of the shift, we show that the threshold corrections which
may exist in some intermediate scale new physics such as supersymmetric
standard model can diminish it, so we can achieve the complementarity relation
at a low energy.
