This paper studies those values
u which can be reached from u0 across a shock or contact discontinuity which may
arise in weak solutions to the Cauchy problem
where it is not assumed that F′′> 0. A condition which guarantees uniqueness is
used to define a set lS(u0) consisting of such values u. The close relationship
between lS(u0) and the geometry of F(u) is examined, and properties of this
set are derived which give information about the interaction of curves of
discontinuity.