Abstract : The focus of this work is the study of interrelations between the different numerical-algebrical and geometrical fields(NAG) - in the teaching of mathematics on the 3rd and 2nd Forms in France.The objective is to understand how the teachers use and make the pupils work on those interrelations and to study how much the use of those interrelations may favour the process of teaching - learning. Relying on the frame of the theory of didactic anthropology developed by Yves Chevallard, we begin with the hypothesis that, there is a didactic void in the interrelations as used as tools, as well as objects linked with the teaching of mathematics in the 2ND level. In spite of this didactic void, the interrelations are present in the practise of the teachers, they have a place and an important role in the teaching of mathematics. This void may represent an obstacle for the pupils when they are supposed to do a resolution of problems which call, at the same time, on the numerical-algebrical and geometrical fields and which are built on new knowledges. The thesis presents teaching practices characteristics about the NAG in the frame of the Observatory of practices linked with numerical initiated by Alain Bronner The methodology is of the clinical type, it relies on the data accumulated by teachers and their pupils in a 3rd and 2nd Classes. The work is made of several parts, about the NAG : a historical and epistemologic study, a study of programs, the study of the pracises of the two teachers and the obvious evidence of the present conditions of the teaching. The reseach has made it clear that, there was a didactic problem which seems not visibly determined, even underestimated by the teachers, about the role of the interrelations between the different mathematic fields.