Vertical and horizontal equity principies occupy the core of income tax systems evaluation. Vertical inequality is measured in terms of relative or absolute income inequality indices, as in the analysis of the redistributive effects (RE) of progressive tax systems. Horizontal equity measurement has been more controversial. Classical horizontal inequities (HI), undestood as unequal treatment of equals or close similars, should be distinguished from reranking (RKG) caused by equity and non equity tax breaks, in actual tax schemes. We propose to integrate the measurement of RE, HI, and RKG in a social welfare framework where tax units non-income differences in needs are recognized. Additively decomposable measurement instruments by population subgroup are found essential to clarify the issues involved
