The dependence of optical polarisation of blazars on the synchrotron
  component peak frequency
  The RoboPol instrument and the relevant program was developed in order to
conduct a systematic study of the optical polarisation variability of blazars.
Driven by the discovery that long smooth rotations of the optical polarisation
plane can be associated with the activity in other bands and especially in
gamma rays, the program was meant to investigate the physical mechanisms
causing them and quantify the optical polarisation behaviour in blazars. Over
the first three nominal observing seasons (2013, 2014 and 2015) RoboPol
detected 40 rotations in 24 blazars by observing a gamma-ray-loud and
gamma-ray-quite unbiassed sample of blazars, providing a reliable set of events
for exploring the phenomenon. The obtain datasets provided the ground for a
systematic quantification of the variability of the optical polarisation in
such systems. In the following after a brief review of the discoveries that
relate to the gamma-ray loudness of the sources we move on to discuss a simple
jet model that explains the observed dichotomy in terms of polarisation between
gamma-ray-loud and quite sources and the dependence of polarisation and the
stability of the polarisation angle on the synchrotron peak frequency.
