  Given a web (multi-foliation) and a linear system on a projective surface we
construct divisors cutting out the locus where some element of the linear
system has abnormal contact with the leaf of the web. We apply these ideas to
reobtain a classical result by Salmon on the number of lines on a projective
surface. In a different vein, we investigate the number of lines and of
disjoint lines contained in a projective surface and tangent to a contact
distribution.
