  Given a simply connected planar domain D, distinct points x,y \in \partial D,
and \kappa >0, the Schramm-Loewner evolution SLE_\kappa is a random continuous
non-self-crossing path in the closure of D from x to y. The
SLE_\kappa(\rho_1;\rho_2) processes, defined for \rho_1, \rho_2 > -2, are in
some sense the most natural generalizations of SLE_\kappa.
  When \kappa \leq 4, we prove that the law of the time-reversal of an
\SLE_\kappa(\rho_1;\rho_2) from x to y is, up to parameterization, an
SLE_\kappa(\rho_2;\rho_1) from y to x. This assumes that the "force points"
used to define SLE_\kappa(\rho_1;\rho_2) are immediately to the left and right
of the SLE seed. A generalization to arbitrary (and arbitrarily many) force
points applies whenever the path does not (or is conditioned not to) hit the
boundary of D except at the endpoints.
  The time-reversal symmetry has a particularly natural interpretation when the
paths are coupled with the Gaussian free field and viewed as rays of a random
geometry. It allows us to couple two instances of the Gaussian free field (with
different boundary conditions) so that their difference is almost surely
constant on either side of the path. In a fairly general sense, adding
appropriate constants to the two sides of a ray reverses its orientation.
