Our study is motivated by the importance of the number line in the visualisation of elementary properties of operations and numbers. For the number line to be an aid for visualisation, first it must be conceived as a coordinate system (origin, orientation and unit). This paper focuses on the construction of the concept of position on a line, and on the number as a means of remembering position, for pre-elementary school children aged 5-6 years. Using didactic design as a methodology, we conducted an experiment to observe how knowledge was constructed while carrying out a certain kind of task in a situation where position had to be designated. Our study shows that pupils, despite recognising number as a quantity, fail to apprehend number as something which represents position. They are, however, capable of understanding the characteristic elements of the number line in suitable situations. Our research suggests that it is possible to teach the use of ordinal numbers to young pupils. Published by Springer http://link.springer.com/ Avaible on line http://link.springer.com/article/10.1007/s11858-013-0554-y?no-access=true