This paper analyses magnetosphere-ionosphere (MI) coupling from a perspective that is independent of inertial reference frame, explicitly acknowledging the role of special relativity in MI coupling. We review the theory of special relativity in the context of MI coupling, and discuss how the MI coupling literature has used a particular low-velocity limit of special relativity known as the “magnetic limit”. We discuss how purely electrodynamic approaches to MI-coupling, where the high latitude electric field plays a central role, depend on inertial reference frame, so descriptions of MI-coupling involving the electric field depend on what reference frame is used to build the physical description. Choosing different reference frames leads to different descriptions of the physics, and essential physics common to all reference frames may be missed by tying the physical description to a specific reference frame. Reference frame-independent descriptions require that ion-neutral relative velocities and ion-neutral collisions are central to MI-coupling. Yet, the literature contains several examples of MI coupling theories that ignore the neutrals and focus instead on the electric field. Whereas neutral wind effects have been reported to modify electrodynamic effects such as Joule heating by ~25%, we show that the consequences of relative motion between ions and neutrals result in much larger impacts for significant geomagnetic storms when ion-neutral velocity differences are largest near the initiation of large-scale ion convection.