The Hammett substituent constant, , is composed of two independent terms: an inductive effect  and a resonance polar effect . These components represent the consequences of the presence of a particular substituent on reactivity through sigma and pi bonds, respectively. For a particular substituent, the value of  is generally assumed to be a constant, irrespective of the nature of the reaction; however, it has been shown that for reactions of para-substituted compounds in which the transition state bears a nearly full charge,  does not remain constant, and thus, the sum  is also variable. In other words, for such reactions, application of the standard Hammett Equation does not produce a linear plot. To correlate these deviations from linearity, Yasuhide Yukawa and Yuho Tsuno proposed a modification to the original Hammett equation which accounts exclusively for enhanced resonance effects due to the high electron demand during such reactions.