  The observed polarization of radio pulsars involves several peculiar effects,
such as comparable amount of orthogonal polarization modes (OPMs) which often
bear the same handedness of circular polarisation V. In the average profiles of
B1913+16 and B1933+16, orthogonal jumps of polarization angle (PA) are observed
to occur at the maximum V, instead of V = 0. High levels of V are also observed
in core components (eg. in B1237+25), where they are accompanied by strong
distortions of PA from the rotating vector model. In weakly polarized emission,
PA jumps by 45 deg are observed in B1919+21 and B0823+26. It is shown that all
these peculiarities can be interpreted in a model which assumes coherent
addition of waves in natural propagation modes.
