According to the framework of the symposium, this presentation will illustrate the use of descriptive categories featuring a teacher & students' joint action model (Sensevy, Mercier, Schubauer-Leoni, Ligozat & Perrot, 2005 ; Schubauer-Leoni, Leutenegger, Ligozat & Flückiger, 2007 ; Ligozat & Leutenegger, in press ; Sensevy & Mercier, 2007) to account for ordinary teaching and learning practices. By ordinary teaching and learning practices, we mean what teachers and students usually do without any specific input from educational research. Our categories aim at revealing how the teacher acts in order to gradually build a common ground in the classroom, and how the students may participate to this teaching intention. To achieve this, we point out (1) the nature of objects and their relationships existing for the participants in the setting (mesogenesis) ; (2) the succession of objects and relationships indicated by participants toward the definition of a common ground (chronogenesis) ; (3); the role / activity of participants toward a given object or relationship (topogenesis). We are particularly interested in the way individual meaning- making processes are constrained by the collective shared intention to teach / intention to learn existing within educational institutions, toward the statement of a collective meaning. More generally, we consider the teacher & students' joint action as a collective form is driven by a typified body of knowledge, conveyed school institutions and teaching materials. Thus, the didactical joint action cannot be regarded solely from the participant's point of view, but the institutional point of view (ie what type of knowledge is intended to taught and learn about a subject) has also to be considered. Our presentation will analyse several episodes taken out of a series of mathematics lessons aiming at teaching measurement of quantities at primary school. The case of surface area measurement as a first approach with 4th graders will be examined through the didactical joint action unfolding over 2-4 lessons. The study will depict the progression of the meaning-making process of a couple of students in different encounters and the effect of the teacher's actions on this meaning-making process, both in the light of the very nature of the knowledge being stated in the common ground from an epistemological point of view. Our modeling of the joint action should help us to notice any mathematical practices emerging from actions within a "physical" encounter.