A quantitative framework for relative rock physics includes a definition of relative elastic property that is rooted in inverse theory and that can be used practically to compute the properties. The framework also includes a set of rules that quantify how operations on absolute properties affect their relative counterparts. From these rules, a comprehensive table of relative elastic properties can be generated, all of which are expressed in terms of relative P-velocity, S-velocity, and density. Finally, the framework includes empirical and model-based rock-physics rules that can be applied directly to the relative properties without reverting to their absolute counterparts. That provides a practical route to such interpretation techniques as direct-to-seismic fluid substitution. The framework offers a quantitative alternative to workflows centered on absolute seismic inversion. In some cases, relative rock-physics workflows are preferable to absolute flows because they avoid statistical complications associated with the nonstationarity of the mean of absolute elastic properties.