 By the early years of the 20th century, the notion of inertial system seems to have been widely accepted as the basis for Newtonian mechanics, even if the specific works of Lange and Thomson were little noticed. In writing “On the electrodynamics of moving bodies” in 1905, Einstein took it to be obvious to his readers that classical mechanics does not require a single privileged frame of reference, but an equivalence-class of frames, all in uniform motion relative to each other, and in any of which “the equations of mechanics hold good.” Two inertial frames with coordinates \((x, y, z, t)\) and \((x', y', z', t')\) are related by the Galilean transformations,