  The slope of the nuclear symmetry energy at saturation density $L$ is pointed
out as a crucial quantity to determine the mass and width of neutron-star
crusts. This letter clarifies the relation between $L$ and the core-crust
transition. We confirm that the transition density is soundly correlated with
$L$ despite differences between models, and we propose a clear understanding of
this correlation based on a generalised liquid drop model. Using a large number
of nuclear models, we evaluate the dispersion affecting the correlation between
the transition pressure $P_t$ and $L$. From a detailed analysis it is shown
that this correlation is weak due to a cancellation between different terms.
The correlation between the isovector coefficients $K_{\rm sym}$ and $L$ plays
a crucial role in this discussion.
