This wave is like the surface reflection PP, except that the reflection occurs at an interface at depth z (expressed in kilometers, e.g., P600P) instead of at the surface. Similarly, for conversions from P to S on writes PzS (or vice versa). The new IASPEI phase name standards additionally differentiate with a + or - sign, whether this reflection/conversion occurred on the outer (upper) side (+) or on the inner (lower) side (-) of a discontinuity; e.g., P660-P is a P reflection from below the 600 km discontinuity, which means this phase is a precursor to PP for these and other such symbols used in seismic phase names see IS 2.1 in this Manual and Storchak et al. (2003 and 2011)