  We describe a scenario to explain blazar periodicities with timescales of
$\sim$ few years. The scenario is based on a binary super-massive black hole
(SMBH) system in which one of the two SMBH carries a jet. We discuss the
various mechanisms that can cause the jet to precess and produce corkscrew
patterns through space with a scale of $\sim$ few pc. It turns out that the
dominant mechanism responsible for the precession is simply the imprint of the
jet-carrying SMBH orbital speed on the jet. Gravitational deflection and
Lense-Thirring precession (due to the gravitational field of the other SMBH)
are second order effects.
  We complement the scenario with a kinematical jet model which is inspired to
the spine-sheath structure observed in M87. One of the main advantages of such
a structure is that it allows the peak of the synchrotron emission to scale
with frequency according to $\nu F\propto \nu^{\xi}$ as the viewing angle is
changed, where $\xi$ is not necessarily 3 or 4 as in the case of jets with
uniform velocity, but can be $\xi \sim 1$.
  Finally, we apply the model to the source PG1553+113, which has been recently
claimed to show a $T_{\rm obs}=(2.18\pm 0.08)\text{ yr}$ periodicity. We are
able to reproduce the optical and gamma-ray light curves and multiple
synchrotron spectra simultaneously. We also give estimates of the source mass
and size.
