From the humble three-legged milking stool to a SEMI standard wafer pod location to numerous sub-micron fixturing applications in instruments and machines, exactly constrained mechanisms provide precision, robustness, and certainty of location and design. Kinematic couplings exactly constrain six degrees of freedom between two parts and hence closed-form equations can be written to describe the structural performance of the coupling. Hertz contact theory can also be used to design the contact interface so very high stiffness and load capacity can also be achieved. Potential applications such as mechanical/electrical couplings for batteries could enable electric vehicles to rapidly exchange battery packs.