  Magnetic Rayleigh-Taylor (MRT) instabilities may play a relevant role in many
astrophysical problems. In this work the effect of magnetic shear on the growth
rate of the MRT instability is investigated. The eigenmodes of an interface and
a slab model under the presence of gravity are analytically calculated assuming
that the orientation of the magnetic field changes in the equilibrium, i.e.,
there is magnetic shear. We solve the linearised magnetohydrodynamic (MHD)
equations in the incompressible regime.We find that the growth rate is bounded
under the presence of magnetic shear. We have derived simple analytical
expressions for the maximum growth rate, corresponding to the most unstable
mode of the system. These expressions provide the explicit dependence of the
growth rate on the various equilibrium parameters. For small angles the growth
time is linearly proportional to the shear angle, and in this regime the single
interface problem and the slab problem tend to the same result. On the
contrary, in the limit of large angles and for the interface problem the growth
time is essentially independent of the shear angle. In this regime we have also
been able to calculate an approximate expression for the growth time for the
slab configuration. Magnetic shear can have a strong effect on the growth rates
of the instability. As an application of the results found in this paper we
have indirectly determined the shear angle in solar prominence threads using
their lifetimes and the estimation of the Alfv\'en speed of the structure.
