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 _ { 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 .