  Rayleigh waves are considered for crystals possessing at least one plane of
symmetry. The secular equation is established explicitly for surface waves
propagating in any direction of the plane of symmetry, using two different
methods. This equation is a quartic for the squared wave speed in general, and
a biquadratic for certain directions in certain crystals, where it may itself
be solved explicitly. Examples of such materials and directions are found in
the case of monoclinic crystals with the plane of symmetry at $x_3=0$. The
cases of orthorhombic materials and of incompressible materials are also
treated.
