  Aims: In the present work we investigate the nature of the magnetic
Rayleigh-Taylor instability at a density interface permeated by an oblique,
homogeneous magnetic field in an incompressible limit. Methods: Using the
system of linearised ideal incompressible magnetohydrodynamics (MHD) equations
we derive the dispersion relation for perturbations of the contact
discontinuity by imposing the necessary continuity conditions at the interface.
The imaginary part of the frequency describes the growth rate of waves due to
instability. The growth rate of waves is studied by solving numerically the
dispersion relation. Results: The critical wavenumber at which waves become
unstable, present for a parallel magnetic field, disappears, due to the
inclination of the magnetic field and instead waves are shown to be unstable
for all wavenumbers. Theoretical results are applied to diagnose the structure
of the magnetic field in prominence threads. When applying our theoretical
results to observed waves in prominence plumes, we obtain a wide range of field
inclination angle; from 0.5 degrees up to 30 degrees. These results highlight
the diagnostic possibility of our study.
