We use the theory of didactic transposition to explain the evolution of the knowledge to be taught around proportionality from classical mathematics up to the present time, paying special attention to the effects of the New Math reform. What happened to be an important element at the core of old school mathematics, the “Theory of Ratios and Proportions”, was intended to be replaced by the new language of sets, variables and functions. After the failure of the New Math reform, some elements of the classical curriculum were partially restored. This gave rise to a hybrid organization where proportions remain disconnected from equations and functions, and where the notion of quantity has trouble finding its place. Through the case of proportionality, we show how didactic transposition appears as a key tool to explain the decisions taken in the constitution of new curricula and their effects on the structure and coherence of the knowledge to be taught, that is, on the efficacy of the knowledge tools students are required to learn.