In this paper, we use the theory of didactic situations to characterize a mathematics teaching practice, currently used in secondary schools in France, which we have called "interactive synthesis discussion." We have studied this practice in ordinary classes, i.e. classes where the researcher intervenes neither in the preparation nor in the management of the lessons. We have looked at the didactic situations the teacher chooses, and how he manages his teaching project, the students' work in the classroom and at home, and classroom interactions. We present two case studies of experienced teachers, one in grade 8, and the other in grade 10.