The recent discovery of the post-perovskite phase transition has great importance for understanding the processes and structures in the lowermost mantle. Using recordings of seismic events that sample the deep mantle, we can test different hypotheses of mantle processes and the state of minerals. In this study, earthquakes which sample different regions of the Earth are used to verify, whether the observed D" reflections (features in the lowest 200-300km of the Earth’s mantle) are due to the post-perovskite phase transition and also to what extent the post-perovskite phase occurs. In some fast-velocity regions, more than one discontinuity is observed, giving rise to a model in which perovskite changes to post-perovskite and back to perovskite at a deeper level. This model has been tested in only a few regions so far, most of them fast-velocity regions in tomographic models. Especially regions of low seismic velocities need to be tested, since here the observations of D" structures are still very diverse and detections in those regions provide a good way of testing post-perovskite as possible source of lowermost mantle reflections. In addition, several regions with D" structures are investigated in detail in order to find the sharpness and polarity of the seismic velocity discontinuities, thereby providing additional constraints on the mineralogy of the lowermost mantle.