Lateral velocity gradients are calculated from the division of the globe into regions of similar Rayleigh-wave group-velocity dispersion characteristics for a 30 sec period carried out by Santo and Sato (1966). The plate boundaries are identified on a lateral velocity gradient scale. Trenches are uniformly characterised by steep lateral velocity gradients whereas in regions around the ridges, Rayleigh waves are non-dispersive in the 20-35 sec period range. It has been found possible to distinguish the trenches caused by oceanic plate-continental plate contacts from those caused by continental plate-continental plate contacts. In both the cases the lateral velocity gradients are high, but in the first case they are due to a rapid increase in lateral velocity from typical continental type of structure to typical oceanic type of structure, whereas in the second case they are due to the rapid decrease in lateral velocity from typical continental type to mountainous type of structure.