The conventional notation ascribed to seismic waves on the basis of their travel times from their earthquake sources. Letters are used to designate the type of wave along the different portions of its travel path: P and S refer to P-waves and S-waves that do not travel through the Earth’s core; K and I refer to P-waves only, which travel through the core and inner core respectively. The letter J refers to S-waves which are generated by the conversion of incident P-waves at the boundary between the inner and outer cores, and which travel only in that mode within the solid inner core. As they propagate towards the Earth’s surface, J-waves are reconverted back into P-waves at the inner core boundary. A repetition of the letters (e.g. PP, SS) indicates that the waves have been reflected at the Earth’s surface; reflection at the outer edge of the core is indicated by ‘c’ (e.g. PcP, PcS). For example, the wave PKIKP travels from the surface, through the mantle to the outer core, into the inner core, and then back to the surface; the wave PPP has travelled only in the crust and upper mantle, and has been reflected twice off the surface.