  PolyChord is a novel nested sampling algorithm tailored for high-dimensional
parameter spaces. This paper coincides with the release of PolyChord v1.3, and
provides an extensive account of the algorithm. PolyChord utilises slice
sampling at each iteration to sample within the hard likelihood constraint of
nested sampling. It can identify and evolve separate modes of a posterior
semi-independently, and is parallelised using openMPI. It is capable of
exploiting a hierarchy of parameter speeds such as those present in CosmoMC and
CAMB, and is now in use in the CosmoChord and ModeChord codes. PolyChord is
available for download at: http://ccpforge.cse.rl.ac.uk/gf/project/polychord/
