  We develop relative oscillation theory for general Sturm-Liouville
differential expressions of the form \[
  \frac{1}{r}\left(-\frac{\mathrm d}{\mathrm dx} p \frac{\mathrm d}{\mathrm dx}
+ q\right) \] and prove perturbation results and invariance of essential
spectra in terms of the real coefficients $p$, $q$, $r$. The novelty here is
that we also allow perturbations of the weight function $r$ in which case the
unperturbed and the perturbed operator act in different Hilbert spaces.
