This case study of a third grade classroom details the ways that competence was jointly constructed by students and the teacher in relation to students' efforts to engage with each other's mathematical ideas. By creating consistent opportunities for students to share and make sense of others' ideas, and by supporting students to take up these opportunities, the teacher expanded shared notions of competent participation. Analyses revealed a large number of student-initiated talk moves, including moves that were available to students even when they might be unsure about the mathematics under discussion. Constructing competence in this way created a range of opportunities for students to participate, distributing authority and encouraging students to take up agentive roles in classroom discussions.