  We study topological defect lines in two-dimensional rational conformal field
theory. Continuous variation of the location of such a defect does not change
the value of a correlator. Defects separating different phases of local CFTs
with the same chiral symmetry are included in our discussion. We show how the
resulting one-dimensional phase boundaries can be used to extract symmetries
and order-disorder dualities of the CFT.
  The case of central charge c=4/5, for which there are two inequivalent world
sheet phases corresponding to the tetra-critical Ising model and the critical
three-states Potts model, is treated as an illustrative example.
