The main interaction of the interior with the lithosphere is as a material source and sink. An absolute reference frame defined by minimizing the translational motion of tectonic plate boundaries differs by 0.6 cm/yr from a frame defined by hot spot traces and by 0.4 cm/yr from the frame defined by the most plausible model of drag forces on the plate. The rms absolute translational velocities are about 2 cm/yr for ocean-ocean plate boundaries and 11/2 cm/yr for ocean-continent plate boundaries. The close agreement between the source and sink and the drag-dependent definitions suggests that the lithosphere, as a stress guide, to some extent controls the locations of its sources and sinks.