effect because they reduce purchasing power to taxpayers. Taxes cause a substitution effect when taxation causes a substitution between taxed goods and untaxed goods. If we consider, for instance, two normal goods, x and y, whose prices are respectively px and py and an individual budget constraint given by the equation xpx + ypy = Y, where Y is the income, the slope of the budget constraint, in a graph where is represented good x on the vertical axis and good y on the horizontal axes, is equal to -py/px . The initial equilibrium is in the point (C), in which budget constraint and indifference curve are tangent, introducing an ad valorem tax on the y good (budget constraint: pxx + py(1 + τ)y = Y), the budget constraint's slope becomes equal to -py(1 + τ)/px. The new equilibrium is now in the tangent point (A) with a lower indifferent curve. As can be noticed the tax's introduction causes two consequences: It changes the consumers' real income (less purchasing power) It raises the relative price of y good. The income effect shows the variation of y good quantity given by the change of real income. The substitution effect shows the variation of y good determined by relative prices' variation. This kind of taxation (that causes the substitution effect) can be considered distortionary. Another example can be the introduction of an income lump-sum tax (xpx + ypy = Y - T), with a parallel shift downward of the budget constraint, can be produced a higher revenue with the same loss of consumers' utility compared with the property tax case, from another point of view, the same revenue can be produced with a lower utility sacrifice. The lower utility (with the same revenue) or the lower revenue (with the same utility) given by a distortionary tax are called excess pressure. The same result, reached with an income lump-sum tax, can be obtained with these following types of taxes (all of them cause only a budget constraint's shift without causing a substitution effect), the budget constraint's slope remains the same (-px/py): A general tax on consumption: (Budget constraint: px(1 + τ)x + py(1 + τ)y = Y) A proportional income tax: (Budget constraint: xpx + ypy = Y(1 - t)) When the t and τ rates are chosen respecting this equation (where t is the rate of income tax and tau is the