  We give the global homotopy classification of nematic textures for a general
domain with weak anchoring boundary conditions and arbitrary defect set in
terms of twisted cohomology, and give an explicit computation for the case of
knotted and linked defects in $\mathbb{R}^3$, showing that the distinct
homotopy classes have a 1-1 correspondence with the first homology group of the
branched double cover, branched over the disclination loops. We show further
that the subset of those classes corresponding to elements of order 2 in this
group have representatives that are planar and characterise the obstruction for
other classes in terms of merons. The planar textures are a feature of the
global defect topology that is not reflected in any local characterisation.
Finally, we describe how the global classification relates to recent
experiments on nematic droplets and how elements of order 4 relate to the
presence of $\tau$ lines in cholesterics.
