"Collective argumentation" occurs when a group works together to arrive at a conclusion (supporting it with evidence). Simplistically, this occurs when students give answers to questions and tell how they arrived at the answer, perhaps prompted by a teacher. But collective argumentation can be much richer, with a focus on the process of solving a problem and why that process is appropriate. In this article, the authors describe a study in which they videotaped and analyzed units of instruction (for at least seven class days) by two teachers who exhibited a diverse range of teaching practices. They used 277 diagrams to represent the arguments that took place during our observations. The study sought to answer the following questions: (1) What parts of arguments were provided by the teacher and by the students? (2) What kinds of questions from the teacher prompted students to contribute parts of arguments? and (3) What kinds of warrants were provided by the teacher and by the students? Herein, they provide an example from the classroom in the study to exemplify the most crucial aspects of the teacher's role in collective argumentation.