The equation in the form: \[\log _{10}(\frac{k}{k_{0}}) = \rho \ \sigma \] or \[\log _{10}(\frac{K}{K_{0}}) = \rho \ \sigma \] applied to the influence of meta- or para-substituents $\ce{X}$ on the reactivity of the functional group $\ce{Y}$ in the benzene derivative m- or $\ce{p-XC6H4Y}$. \(k\) or \(K\) is the rate or equilibrium constant, respectively, for the given reaction of m- or $\ce{p-XC6H4Y}$; \(k_{0}\) or \(K_{0}\) refers to the reaction of $\ce{C6H5Y}$, i.e. $\ce{X} = \ce{H}$; is the substituent constant characteristic of m- or $\ce{p-X}$: is the reaction constant characteristic of the given reaction of $\ce{Y}$. The equation is often encountered in a form with \(\log _{10}k_{0}\) or \(\log _{10}K_{0}\) written as a separate term on the right hand side, e.g. \[\log _{10}k = \rho \ \sigma +\log _{10}k_{0}\] or \[\log _{10}K = \rho \ \sigma +\log _{10}K_{0}\] It then signifies the intercept corresponding to $\ce{X} = \ce{H}$ in a regression of \(\log _{10}k\) or \(\log _{10}K\) on \(\sigma \).