  We present a version of the conditional symmetry method in order to obtain
multiple wave solutions expressed in terms of Riemann invariants. We construct
an abelian distribution of vector fields which are symmetries of the original
system of PDEs subjected to certain first order differential constraints. The
usefulness of our approach is demonstrated on simple and double wave solutions
of MHD equations. The paper also contains a comparison of the conditional
symmetry method with the generalized method of characteristics.
  Received 19 April, 2003; Accepted 10 June, 2003
  Final Version published in J. Nonlinear Math. Phys. 11,1 47-74, (2004).
