The « rule of three » is a basic tool in proportionality problems, but its place in French education and its presentation have varied throughout time. The origin of the name is also uncertain: some see it in the fact that three stages are needed to write out the reasoning for the proportionality using the unitary method, others see the fact that three quantities are known and the fourth is to be found. In classes of students from 12 to 14 years of age, the reading of extracts from Indian texts in Sanskrit in which the rule trairâshika (relating to three quantities) is used, shows the algorithmic aspect of this rule. However, in the Indian texts, the values involved in the calculations had very specific names, which reflected the underlying reasoning of proportionality, whether it was for the basic rule of three or the associated rules. These names have not been retained in the handing down of texts in the West and the formulae have become less precise.