occur only along the last closed field line in the plane of the dipole and rotation axes. Because the charged particles of both positive and negative charges are accelerated in the gap, the emissions should beam both toward and away from the pulsar. Therefore, the observed fan beams consist of those coming from different gaps and the measured phase separation between the two peaks is determined by the time travel difference between these two outer gaps, relativistic aberration of emission and the bending of the magnetic field lines near the light cylinder. The emission from each peak is highly cusped because of the relativistic aberration, so there will be some bridge emission but very little other offpulse emission. Obviously, the pulse profiles of CHR model are not consistent with the observed those. So the three dimensional description of the outer gaps is necessary.

After studying the $\gamma$-ray production and light curves for various magnetosphere geometries based on the CHR model, Chiang and Romani (1992) assumed that gap-type regions could be supported along all field lines which define the boundary between the closed region and open field line region rather than just on the bundle of field lines lying in the 