Considering the growing number of observations of a seismic discontinuity at the top of D', some 300 km above the core-mantle boundary, we investigate the scenario of whether this feature could be global. We show that the mean first arrivals of P-waves, as used to build global models, can be fit by the two branches of a discontinuous model. We argue that the intermittence of the detection of the reflector is due to lateral variations of a global discontinuity. These observations put strong constraints on the type of chemical layering that could explain the discontinuity. We speculate that the properties of the D' discontinuity are better explained in terms of a new phase transition. ¿ American Geophysical Union 1993