Abstract
The 1971,1973 Tax Reform decided to regulate the relation between business income and result of the income statement according to the model of the "partial derivation". This decision was confirmed by the 2003 Reform, that increased strongly the number of adjustments to the result of the income statement allowed or prescribed, steering the system toward the "double rail" model. This trend did not last long. With the Financial Act for 2008 it was abandoned. and the system started moving in the opposite direction, toward the "single rail" model. This essay, after a description of the stages of this process, analyzes the main advantages and disadvantages of the present regulatory framework.