The modern theory of price discrimination has widely inherited from the work of Arthur Cecil Pigou (1920) and of Joan Robinson (1933). Devoting two chapters of her book The Economics of Imperfect competition to the issue of third degree of discrimination, Robinson examined the necessary conditions to discrimination (market power, products homogeneity, different elasticity of consumers demand to prices, absence of demand and goods transfers) and presented a graphic analysis of discriminate monopoly decisions, which have been included in microeconomics manuals. Price discrimination is defined as the action of selling a same article, with a same control, at different prices to different consumers. Then, to discriminate means to apply high prices to some consumers but also to fix lower prices to others