This chapter describes part of a research and development project in school mathematics teaching. It covers a series of eight lessons that occured in the fall 1987. It is about communication in a mathematics classroom and provides evidence for the argument that students can actively contribute to constructing mathematical meaning while at the same time gaining experience with the conventions that enable discourse in the discipline. The focus is on the role a classroom teacher might take in supporting the development of students' ideas and relating them to mathematical traditions.