  The truncation of stellar discs is not abrupt but characterized by a
continuous distancing from the exponential profile. There exists a truncation
curve, $t(r)$, ending at a truncation radius, $r_t$. We present here a
theoretical model in which it is assumed that the magnetic hypothesis
explaining the flat rotation curve also explains the truncation. Once stars are
born, the centripetal magnetic force previously acting on the progenitor gas
cloud is suddenly interrupted, and stars must move to larger orbits or escape.
The agreement between theoretical and observed truncation curves is very
satisfactory. Parameters defining the disc gas rotation curve should therefore
be related to those defining the truncation. It is predicted that rotation
curves that quickly reach the asymptotic value $\theta_0 = \theta (r=\infty)$
would have small truncation radii. On the contrary, $r_t$ and $\theta_0$
itself, would be uncorrelated quantities.
