Various equations are associated with R.W. Taft, but the term is most often used to designate the family of equations that emerged from Taft's analysis of the reactivities of aliphatic @E02219@, and which involved the polar substituent constant \(\sigma \text{*}\) and the steric substituent constant \(E_{\text{s}}\): \[\log _{10}k = \log _{10}k_{0}+\rho \text{*}\ \sigma \text{*}+\delta \ E_{\text{s}}\] or the one-parameter forms applicable when the role of either the polar term or the steric term may be neglected. Nowadays \(\sigma \text{*}\) is usually replaced by the related constant \(\sigma _{I}\).