  The relation between the slope of the nuclear symmetry energy at saturation
density and the neutron skin thickness is investigated. Constraints on the
slope of the symmetry energy are deduced from the neutron skin data obtained in
experiments with antiprotonic atoms. Two types of neutron skin are
distinguished: the "surface" and the "bulk". A combination of both types forms
neutron skin in most of nuclei. A prescription to calculate neutron skin
thickness and the slope of symmetry energy parameter $L$ from the parity
violating asymmetry measured in the PREX experiment is proposed.
