ABSTRACT
This chapter presents a case of the students and teacher in a sixth-grade classroom engaged in developing concepts of justification and proof. Specifically, the case focuses on the evolution of how the students in the class justified their conjectures about the commutative property of multiplication. The case examines tasks and interactions in which the students and teacher jointly developed socio-mathematical norms for what would count as justification and considers the reflexive relation between the classroom culture in which these norms evolved and the conceptions of individual students who participated in constructing them.
