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 esters, and which involved the polar substituent constant σ * and the steric substituent constant E 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 σ * is usually replaced by the related constant σ I.