Kinetic energy in present-day plate motions in partitioned approximated equally between poloidal and toroidal parts. The poloidal part is excited by buoyancy forces and is associated with cellular flow in the mantle. Most of the toroidal part is derived from strike-slip motion at transform faults and from oblique subduction at convergent margins. Plate spin accounts for only a few percent of the toroidal energy of the lithosphere. Using a simple kinematic model, we demonstrate that a square-shaped plate, drifting without spin, has equal poloidal and toroidal energies. We then show that the most probable state for an array of many rectangular plates is equipartition of poloidal and toroidal kinetic energy, provided the drift directions of individual plates in the array are random. Our model predicts that convergent margins occupy 42% of plate boundaries, while divergent and transform margins occupy 25% and 33% respectively. This distribution is in good agreement with the actual distribution along the boundaries of the Pacific, India, Cocos and Nazca plates, which contain more than 90% of the lithospheric kinetic energy. According to these calculations, equipartitioning of plate kinetic energy implies a large degree of decoupling between plates and a disorganized pattern of convection in the mantle. ¿American Geophysical Union 1991