This paper aims at discussing a very popular teaching aid, the so-called number line, where whole numbers are introduced as labels on unit marks by means of a measuring process and where additions and subtractions can be realized, as operators, with jumps forwards and backwards. Traces of this early approach can be found in the teaching practices of most Western countries, but, surprisingly, not in the most popular Chinese textbooks. A question arises: where does the difference come from? In the following, I review some Western literature to sketch out the analysis of the number line as a teaching aid, from the historic-epistemological, cognitive and didactical perspectives. Later some paradigmatic practices from different countries are presented.