  Using a Fermi-liquid-based theory we calculate the in-plane anisotropy of the
spin susceptibility for hole-doped high-T_c cuprates. Employing the
two-dimensional one-band Hubbard model and a generalized RPA-type theory we
consider anisotropic hopping matrix elements ($t_x \neq t_y$) and a mixing of
d- and s-wave symmetry of the superconducting order parameter in order to
describe orthorhombic superconductors. We compare our calculations with
available inelastic neutron scattering data on untwinned YBa{2}Cu{3}O{7-\delta}
and find good agreement. Furthermore, we predict a strongly anisotropic
in-plane dispersion of the resonance peak.
