The linear free-energy relation of the form: \[\log _{10}(\frac{k}{k_{0}})=s\ n\] applied to the variation of reactivity of a given electrophilic substrate towards a series of nucleophilic reagents. n is characteristic of the reagent (i.e. a measure of its nucleophilicity) and s is characteristic of the substrate (i.e. a measure of its sensitivity to the nucleophilicity of the reagent). A scale of n values is based on the rate coefficients k for the reaction of methyl bromide with nucleophiles in water at 25°C, s being defined as 1.00 for these reactions and n being defined as 0.00 for the hydrolysis of methyl bromide. (Other scales have been devised.)