  Polarisation of radio pulsar profiles involves a number of poorly understood,
intriguing phenomena, such as the existence of comparable amounts of orthogonal
polarisation modes (OPMs), strong distortions of polarisation angle (PA) curves
into shapes inconsistent with the rotating vector model (RVM), and the strong
circular polarisation V which can be maximum (instead of zero) at the OPM
jumps. It is shown that the existence of comparable OPMs and of the large V
results from a coherent addition of phase-delayed waves in natural propagation
modes, which are produced by an incident linearly polarised signal. The
coherent mode summation implies opposite polarisation properties to those known
from the incoherent case, in particular, the OPM jumps occur at peaks of V ,
whereas V changes sign at a maximum of the linear polarisation fraction L/I.
These features are indispensable to interpret various observed polarisation
effects. It is shown that statistical properties of the emission mechanism and
of propagation effects can be efficiently parametrised in a simple model of
coherent mode addition, which is successfully applied to complex polarisation
phenomena, such as the stepwise PA curve of PSR B1913+16 and the strong
distortions of the PA curve within core components of pulsars B1933+16 and
B1237+25. The inclusion of coherent mode addition opens the possibility for a
number of new polarisation effects, such as inversion of relative modal
strength, twin minima in L/I coincident with peaks in V , 45 deg PA jumps in
weakly polarised emission, and loop-shaped core PA distortions. The empirical
treatment of the coherency of mode addition makes it possible to advance the
understanding of pulsar polarisation beyond the RVM model.
