In seismic ray path terminology, phases such as PKS and SKP describe waves that travel from a surface-focus earthquake to the core, undergo a wave-type conversion at the core boundary (P-to-S for PKS, S-to-P for SKP), and continue to the observatory; by the seismic reciprocity theorem, phases that involve a swapped sequence of P and S wave-type legs along a ray path connecting the same source-receiver pair travel via genuinely different geometric ray paths (since P and S waves travel at different velocities and therefore refract differently according to Snell's law along the path) but nonetheless arrive with identical total travel times, due to the underlying symmetry of the reciprocity principle.