Synchrotron White Beam X-ray Topography (SWBXT) and synchrotron X-ray anomalous scattering have been employed to determine the polarity of {111} edge facets, anchored to the three phase boundary (TPB) on which twinning is observed to nucleate in Magnetic Liquid Encapsulated Czochralski (MLEC) grown sulfur doped, <001> InP single crystals. Analysis of the results indicates that both the formation of edge facets and the nucleation of twins occur preferentially on {111}P faces. Of the four possible sets of edge facets, belonging to the {111}P form, which are oriented so as to be thermodynamically favored to be anchored to the TPB, two can give rise to a {115} to {111}P external shoulder facet conversion upon twinning, while the other two can give rise to a {114} to {110} conversion. For these cases, twinning is only observed when the {111}P edge facets are anchored to the TPB in a region where the shoulder angle is close to 74.21° or 70.53°, facilitating the production of the {115} and {114} external shoulder facets, respectively, prior to twinning. These observations are discussed in light of calculated surface energies of the various internal and external facets.