According to the IASPEI standard phase name list Pn is P wave bottoming in the upper mantle or an upgoing P wave from a source in the uppermost mantle (see IS 2.1 in this Manual and Storchak et al. (2003 and 2011). This differs to some extent from earlier related definitions and considerations. Beyond a certain critical distance, usually beyond 100 to 200 km depending on crustal thickness and source depth, the first arrival from seismic sources in the crust corresponds to waves refracted from the top of the mantle. The amplitudes of Pn waves are relatively small, with long-period motion followed by larger and sharper waves of shorter period called Pg, which propagated directly through the crust. Yet, at epicentral distances between some 300 to 600 km the Moho reflected PnPn may be the first strong arrival after Pn with sometimes rather large amplitudes. The Pn wave has long been interpreted as a head (conical) wave along the interface between two homogeneous media - namely, crust and mantle. The observed amplitude, however, is usually greater than that predicted for head waves, implying that the velocity change at the crustal base (Mohorovičić discontinuity; Moho) is not exactly step-like but has a finite gradient at or below the transition zone. The designation Pn is commonly applied to short-period P waves that propagate over considerable distances (even up to 20° in continental platform regions) with horizontal phase velocities in the range of ~7.5-8.5 km/s. This can also not been explained in terms of head wave propagation at the Moho (although the horizontal velocity and travel times would agree with it), because head waves must decay rapidly with distance. More likely is an explanation in terms of guided waves, within a highQ layer several tens of kilometers in thickness at the top of the mantle. See Sn; also record examples in Chapter 2 (Figs. 2.18 and 2.40??), Chapter 11 (Figs. 11.41, 11.44. 11.46)??? and DS 11.1 of this Manual, Kulhanek (2002, p. 335-338), and Mooney et al. (2002). 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)