Term used in seismology for “branching” travel-time curves that are related to discrete ray paths of the same type of wave and due to strong velocity gradients and/or low-velocity layers in the Earth’s interior. For a prograding travel-time branch the rate of travel time t increase with increasing epicentral distance D (in km) (or ∆ in degree), i.e., of dt/dD = ray parameter p = horizontal component of slowness s R is decrasing, in contrast to a retrograde (or receding ) travel-time branch for which this rate of change is increasing. This is equivalent with dD/dp < 0 for prograde and dD/dp > 0 for retrograde branches. Thus, where the prograde and retrograde branches merge the sign of dD/dp changes sign the wave energy is focussed, resulting in strongly increased amplitudes (caustic point; See also: Figs. 2.28b and 2.29 in Chapter 2 of this Manual). Retrograde branches may be formed by strong positive velocity increase with depth. When this gradient weakens again at larger depth, a new prograde branch is forming. Such a sequence of prograde, retrograde and prograde travel-time branches in the order of decreasing dt/dD = p for the same wave type is called a triplication. Examples are the two P wave triplications due to the upper mantle discontinuities (strong velocity gradient zones) at 410 km and 660 km depth (See:Figs. 2.29 and 2.5.1 in Chapter 2 of thes Manual) and the PKP triplication with the branches PKPab, PKPbc, PKiKP and PKPdf (Fig. 11.59??? in Chapter 11) due to the strong velocity increase from the outer to the inner Earth core (See:Fig. 2.53??? in Chapter 2. For this and other such symbols used in seismic phase names see IS 2.1 in this Manual and Storchak et al. (2003 and 2011)