In their 1959 publication, Yukawa and Tsuno attributed observed deviations from Hammett Plot linearity in electrophilic reactions to additional resonance effects occurring through the pi bonds of substituent groups in their compounds. This implied that the inductive component of the Hammett substituent constant remains constant in such reactions, while the resonance component, , does not. From this assumption, the two scientists defined a new resonance substituent constant, , that is mathematically represented as follows:
for a reaction in which positive charge is built up at the reactive center in the transition state. In order to quantify the extent of the observed enhanced resonance effects, Yukawa and Tsuno introduced an enhanced resonance parameter, , that quantifies the "demand for resonance" at the reactive center. Thus, the resultant Yukawa–Tsuno effective substituent constant is given by:
and the Yukawa–Tsuno equation (modified Hammett equation) takes the form:
Values of  have been determined and catalogued for a number of substituents for quick application of the Yukawa–Tsuno equation.