  Radio pulsar polarization exhibits a number of complex phenomena that are
classified into the realm of `beyond the rotating vector model' (RVM). It is
shown that these effects can be understood in geometrical terms, as a result of
coherent and quasi-coherent addition of elliptically polarized natural mode
waves. The coherent summation implies that the observed tracks of polarization
angle (PA) do not always correspond to the natural propagation mode (NPM)
waves. Instead, they are statistical average of coherent sum of the NPM waves,
and can be observed at any (and frequency-dependent) distance from the natural
modes. Therefore, the observed tracks of PA can wander arbitrarily far from the
RVM, and may be non-orthogonal. For equal amplitudes of the NPM waves two pairs
of orthogonal polarization modes (OPMs), displaced by 45 deg, can be observed,
depending on the width of lag distribution. Observed pulsar polarization mainly
results from two independent effects: the change of mode amplitude ratio and
the change of phase lag. In the core region both effects are superposed on each
other, which can produce so complex behaviour as observed in the cores of PSR
B1933+16, B1237+25 and J0437-4715. Change of the phase lag with frequency $\nu$
is mostly responsible for the observed strong evolution of these features with
$\nu$. The coherent addition of orthogonal natural waves is a useful
interpretive tool for the observed radio pulsar polarization.
