 In GTR, spacetime is locally very much like STR’s flat Minkowski spacetime. There is no absolute velocity locally, but there are clear local standards of accelerated vs non-accelerated motion, i.e., local inertial frames. In these ‘freely falling’ frames bodies obey the usual rules for non-gravitational physics familiar from STR, albeit only approximately (this is sometimes called the ‘strong equivalence principle’, and is discussed further in section 4 below). But overall spacetime is curved, and local inertial frames may tip, bend and twist as we move from one region to another. The structure of curved spacetime is encoded in the metric field tensor gab, with the curvature encoding gravity at the same time: gravitational forces are so to speak ‘built into’ the metric field, geometrized away. Since the spacetime structure encodes gravity and inertia, and in a Mach-heavy theory these phenomena should be completely determined by the relational distribution of matter (and relative motions), Einstein wished to see the metric as entirely determined by the distribution of matter and energy. But what the GTR field equations entail is, in general, only a partial-determination relation.