This paper draws an Activity Theoretical frame specific to mathematics at school with reference to both Vygotskian and Piagetian approaches. At a local point of view, the frame is oriented toward analysis of students' mathematical activities in the classroom. This local point of view is extended to a global point of view, to gain access to what happen in the classroom, linking students and teachers' activities and the context of the whole activity system. This Activity Theoretical frame is named "Double Approach", referring both to didactical tools coming from the field of didactics of mathematic and also to an ergonomic approach, coming from cognitive ergonomics. (Contains 2 footnotes.)