A promising family of thermoelectrics is based on rocksalt structured PbTe. To explain the thermal conductivity at high temperatures it is necessary to understand why the lead atoms move anomalously from their octahedral sites. This has been attributed to the formation of fluctuating local dipoles, anharmonic phonon behaviour or even challenged based on PDFFIT analysis, energy calculations and EXAFS results, respectively. This proposal will use RMCprofile analysis of higher-quality total scattering data over a wider range of temperatures than previously measured to determine unambiguously the nature of the disorder in terms of its magnitude and directionality and, crucially, whether the disorder is correlated throughout the lattice. This will resolve the debate about the preferred direction of lead atom movement and its similarity or otherwise to related ferroelectric phases.